You'd have the exact same advantage if you ran some random Rock Polish user that can outspeed everything after just one move. Besides, how often do you think you'd be able to actually paralyze everything on the opponent's team? If your opponent has a 75% chance of attacking even after being paralyzed, then chances are your paralysis inducer will be dead before they can paralyze the entire opposing team. Sleep isn't like that. If something is put to sleep, it's completely useless for at least one turn (if not more), giving you a much better chance at setting up, phazing the opponent out to put anther Pokemon to sleep, etc.
No, the chances of winning based on getting free turns with paralysis are minimal, seeing as how you only have a 25% chance each turn of doing so. You're really underestimating the importance of those free turns and the chance you'll actually get them.
Sleep isn't about chance. That's the whole point. Sleep is too easy to exploit because unlike paralysis, there's little to no risk involved. If the opponent is asleep, they're asleep. And no, we shouldn't just ban Serene Grace Jirachi just because there is some luck involved. According to the
Characteristics of a Desirable Pokemon Metagame, we should only ban luck when absolutely necessary (i.e. when it breaks the game). Jirachi with Serene Grace has never once broken the game, and although it has been contested several times, such a ban has never received enough support from the community to even be suspected.
Almost every decent action that can be performed in a game can "provide and advantage". It's the particular advantage and the extent of it that matters, and no advantage matches the potency of the sleep status.
The premise of the sleep clause is sleep itself, not just making battles easier to win. Even if you wanted to paralyze a whole team to run through it with a faster Pokemon, you'd still have to get through the entire team when they can move freely 75% of the time, which is nearly impossible. Besides, your example is no different than if I softened up an opponent's team with Specs Keldeo just so I could clean it up with SD Scizor's Bullet Punch. Should we ban Specs in that case? or priority? No, because that would require skilled playing on my part to get the opposing team weak enough to clean up with SD Scizor. That's what the mid and end games are all about. But sleep isn't something that you have to be skilled about in order to achieve a winning condition. You literally just hit a button and BAM, free turns.
Do you ever wonder why people have the term "sleep fodder" and not "paralysis fodder"? It's because sometimes people will have a certain Pokemon for handling a sleep inducer (such as Breloom), but that particular check or counter might be easily defeated if put to sleep. So people have sleep fodders that they'll switch into the sleep move, something that they don't mind losing, and it's because they know they'll probably end up losing a Pokemon to it. You see, a sleep inducer like Breloom can make a potential counter like Amoonguss completely useless while they set up. Paralysis can't do that, though. You say that paralysis makes a Pokemon useless because it makes them slower, but that's simply not true. Even Pokemon that rely on their speed to be effective can still be a threat if they can get a hit in now and then, so even they aren't complete dead weights. Then there are Pokemon that do not rely on their speed stats at all, which couldn't care less about the speed drop. In that case, you effectively just hit them with Flash, which is hardly even comparable to sleep.
This is a similar argument that was used in the test of Sand Veil/Snow Cloak, but I'll address it again here. Jirachi with Iron Head is not the same as the free turns that sleep would offer. If you get a free turn due to sleep (like you did with a Sand Veil/Snow Cloak miss), you can do whatever you want with that free turn. You can attack the opponent, set up, heal your Pokemon, whatever, Jirachi with Iron Head cannot do that, though, because his variety of hax is active, not passive. He is forced to spam a fairly weak STAB move with poor coverage just to attempt a flinch, not to mention it completely fails against faster Pokemon.
The creation of the Sleep Clause was not subjective in the same manner that such a change as you have proposed would be. When the Sleep Clause was created, people saw just how powerful sleep was and decided to restrict it in some way. The most logical thing is to just limit it to one Pokemon at a time for starters, and even now we've seen through experience that just one Pokemon at a time is still a powerful thing. On the other hand, you're suggesting we give an exception to certain Pokemon based upon whether they are too "good" or not. That is where the extreme subjection comes into play. How do we decide which Pokemon are "bad" enough to get an exception? You mentioned Venomoth, but there's no chance on Earth that you can give a Pokemon with decent offenses, Quiver Dance, and Tinted Lens the ability to put an entire team to sleep and it still not be broken.
But by that logic, should we ban wall breakers as well? Is their main purpose not to soften up the bulkiest members of an opponent's team so that a designated sweeper can clean up late game? Doesn't that bit of damage matter as well? The difference is that those benefits are something that you have to work towards. You don't just hit a button and get free turns. That takes careful prediction and planning.
And what if that Pokemon doesn't care about the speed drop? If you used Thunder Wave on a Forretress or Ferrothorn, you might have just strengthened its Gyro Ball and done little to hurt it otherwise besides a 25% chance of paralysis. Even if you use it on an offensive Pokemon like Terrakion, that Pokemon is still usable. I might switch Terrakion into your Volcarona (assuming you're not running Giga Drain or HP Ground) and you might switch out, giving me a 75% chance of tossing out one of my powerful STAB moves and nailing a switch-in. On the other hand, if my Terrakion were asleep, that would give you a completely free turn to switch to whatever you want, plus more possible free turns after that. Even with an offensive threat, you still aren't matching the restrictive power of sleep with paralysis.
But what would this accomplish exactly? If you really think that Pokemon like Watchog and Butterfree are in lower tiers simply because they can't sleep more than one Pokemon at a time, then you're sadly mistaken.