The other example, where they tag characters for certain roles is that queue for support and want to play Elise support, but she doesn't have a support tag so I'm stuck playing a champ I don't want to play even though Elise is a totally fine support pick these days. Riot determines the champ pool I can play, not a fluid meta.
I think you have a vastly different idea of how a queue would work. It wouldn't lock you into a champion at all. It simply reduces the "who types fastest" in chat part of champ select, and gets 5 people who would have asked for different roles together. You can still play whatever champ you want in that spot. It'd just be an additional layer to their current matchmaking system. Queuing for support might be faster then queuing for mid, but queuing for 'all' would still be faster then anything. If you do that, you roll the dice on what everyone else wants to play. If someone does that, gets put into jungle, and then refuses to jungle.. it's no different then the current system where everything but jungle gets called, and the last guy refuses to jungle.
Seriously, every specific downside example you can think of already exists under the current system. They are not specific to having a role queue, they already happen ALL THE TIME! A role queue would simply reduce how often it happens, which is why it's an improvement.
You can also still queue without choosing a role, and it would be exactly the same as before. All it would do is reduce the random chance that you get into a game and the role you want is open. I want to add, my system would just have 'top, mid, bottom, jungler' so you are just calling a lane, not a role per se. Bottom might have a sub-system that you specify support or carry, or it would still be dependent on who is higher on the pick order, not sure really, haven't sat down and given enough thought to that scenario. But, at least if you queue for 'bottom', you have a better chance of getting your role or one you are semi-familiar with (most ADCs can play support decently and vice versa, imo). As it currently stands, you might want to be ADC, but if that gets called and your forced into top as the last open spot, you may completely suck at top and the game is a loss.