Tbh it doesn't really limit what people play at all, in League for example (where this exact system is in place, and even includes character bans) one-tricking is not just effective, but encouraged & still extremely common. All it does is involve the concept of matchups, you can still play whatever you like but there will be matchups that are harder to play against, this means it's useful to be flexible in SOME cases and you might have an actual desire to play something else. Overwatch is another example, Genji was so counterable for the longest time yet one-tricks that were good enough could still make it work - also emphasises skill expression which is something Valorant is missing.
Also, this wouldn't be emphasised for every character. For example, Sova is an agent that is not overly OP, nor is he purely a ranked agent - he does not need a hard counter because he's fairly standard. My main issues are when in ranked people are only picking Reyna, Chamber, Clove for most games. It just makes the game so stale and repetitive
Also the whole team can still hover their agents, a lot of my ranked games we manage to come to a consensus on who plays what without instalocks. And as for your argument, people are already limited by what they play just cause of the meta - Harbor, Tejo etc. are so bad rn. All I'm suggesting is dependant nerfs, rather than universal nerfs - there will be some agents that are good 'blindpicks', and some that are not