I used to play quite a bit of Teemo in the past (a few matches this year), and I don't really agree with the accusations you're giving or the supporting reasons that you've mentioned. Teemo is by no means a weak champion, he's actually super strong in many ways that players don't think about. The biggest issue regarding any champion in League isn't the champion, but instead the player base's mentality that everything "needs to be able to fight". The reason it applies to Teemo is because, fighting directly isn't where Teemo excels.
Teemo is a champion who has a lot of hidden power in his kit. By hidden, I'm mostly trying to say that it doesn't feel noticeable to the player, but it definitely exists. You as a player might look at Teemo and think "I can't fight anything, I'm not fast enough, this champion sucks".. but then statistically after the game you look and will begin to notice that you had the highest damage in the game, the highest vision score, and the highest crowd control score. That's a typical game from Teemo, and while it feels like you aren't doing enough, you're really doing far more than it feels like.
Anyways, if Riot were to adjust Teemo's W, other things in his kit would have to be removed or lowered to compensate. All of Teemo's abilities are over-tuned already to help make up for his W's lack of power.. or rather, it would more likely be the other way around. Teemo's other abilities are so powerful that Riot really cannot afford to give him anything better on that skill slot.
In case you don't understand, consider what is actually in Teemo's kit:
- Teemo's passive let's him turn INVISIBLE. Where as, most champions use camouflage which can be spotted with wards, nothing will reveal Teemo. Not only can he turn invisible, he can literally move around in a bush without anyone being able to reveal him (except a select amount of champions that can reveal invisible targets like Corki or Lee Sin). For a passive that is actually super powerful despite how weak it might feel for someone who just likes fighting.
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- Teemo's Blinding Dart is a very long CC with a low cooldown and high base damages with also high scaling. To a champion who relies on auto attacks, Teemo's blind is the equivalent of a point and click Morgana bind at a lower cooldown. It might not feel that strong, but trust me.. IT IS.
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- Teemo's Toxic shot is actually more powerful than Singed's poison and it doesn't use any mana --> 34 / 68 / 102 / 136 / 170 (+ 70% AP) per hit, compared to Singed's 40 / 60 / 80 / 100 / 120 (+ 80% AP) damage in 2 seconds.
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- Next we have his ultimate.. Teemo's Noxious Trap is essentially a global ultimate (since he doesn't have to be near a mushroom for it to deal damage) and gives vision like wards with no limit on how many he can place. When it detonates it deals AoE damage per second (with fairly decent base damage and scaling), slows, and gives Teemo vision of his target.. That's not even to mention they have a ranged method of placement which gives Teemo even more safety.
Teemo has 4 massively strong abilities in his kit already. If you think his Move Quick is too weak then there's good odds that you just aren't utilizing his "hidden assets" enough. Even if that isn't the case, there are good chances that you simply don't realize how powerful Teemo's kit really is. His strengths don't involve straight up fighting other players. He's a champion who is designed to be strategic by using his map control, blind, slow, and speed boost to help create advantages for himself.