This sounds fantastic. So much of the stagnancy issues in pro play come from pros picking only a handful of flavor of the month champions and hard-focusing them during practice, because they know they're going to be the only champions that will truly see play in competitions. The difference doesn't have to be that large, and sometimes it might not even be an objective power differential (many OP champions only end up being "discovered" weeks or months after whichever changes made them strong), but the current state of competitive League means that there is no real point in practicing champions that feel weaker than others unless they're meant to be surprise picks. I think this kind of change could help, and I feel there are enough champions out there for elimination mode to be extended throughout an entire competitive bout, e.g. 5+ or even 10+ matches.
Not only would a change like this break the relative stagnancy of the competitive scene, it could also introduce a new element of strategic depth to pro play: with this kind of change, teams would have to ask themselves whether to save up on their best champions until later, or whether to play it safer in the meantime and pick one or more top-tier champions for their current match. We'd get to see wackier synergies and riskier picks on top of the existing compositional standards, as well as completely different strategies in and out of matches.
I also feel a change like this could be inspirational to the rest of us players: as much as many of us hate to admit it, our opinions of champions and the game are heavily influenced by pro play, and often a champion can feel trash simply because they weren't picked too often in recent competitions, even if they're objectively quite good. Pro tactics and strategies also get copied out of competitive play, and a greater wealth of those on display could inspire players to pay more attention to context and try out more things.
The only potential downside to this is that, since we'd be seeing pros play champions they're less familiar with, the quality of their play might end up reduced as a result. However, even that might be a good thing, since pros currently push many champions to their breaking point (Azir and Kalista both rose and fell hard due to their impact on pro play, for example), and focus too hard on only a small portion of the game's potential roster, which also leads to a degree of resistance to new champion releases and strategies. Seeing a pro make mistakes on a champion they haven't spent 100+ hours practicing in the past month is something we can all relate to, and could also help add a degree of unpredictability to every competitive game, since no pro player could afford to become literally flawless at a single champion when they might only be able to play that champion once per bout.