Such a myopic response.
No, it would NOT render them useless. Teemo could still place them in bushes, preventing them from being spotted unless you likewise enter the bush it's in. And, if the shroom is placed in a smart place within the bush, there would be zero chance to react to destroy it before triggering it. Even if attack-moving into the bush.
Good players will recognize this and treat every bush in the jungle as a potential death trap. But it's possible with focused micro to navigate through the jungle and not go through a single bush.
The present stealth detection mechanics make Teemo's shrooms anti-fun and overpowered, because the only useful counterplay to them,
has a 60 second cooldown. This forces a team to either ALL buy sweeping lenses, or to completely avoid stepping foot in the jungle and remain in lanes using minions for cannon fodder for fear of accidentally setting off a trap and losing 50% of their HP (assuming Teemo itemized properly to maximize the DPS of his shrooms)
My proposed change would mean Teemo's shrooms would at the very least be plainly visible in open sight if not placed carefully. Their potency wouldn't be reduced, as anyone that steps on a shroom that they clearly saw out in the lane deserves to take that kind of damage. But at the same time, it would allow his opposing team to still roam through their own jungle so long as they avoided stepping in the bushes.