T9-T11 feels like it was intended as a personal challenge, something you clear for bragging rights mostly.
Also keep in mind that clearing T8 solo can be a challenge for a lot of people (not everyone is a god tier pro player), putting gear behind a T11 would probably mean it would end up being out of reach for those, which is not something you would want to see in content that was meant to be for “everyone” to enjoy.
I’d say this would be more of a “player” problem (once again, not everyone is pro and there is nothing wrong with that), not the class being unable to clear it, you have to press more buttons in a T11 than in T8, you may have to plan your pulls, utilize CCs your class has at disposal and maybe even get consumables.
That is perfectly fine, considering the game calls it “unfathomable challenge” in the tooltip.
As I said before, some players will struggle even with T8, grouping up with others for T11 would eventually end up in their inexperience showing, bricking the run and people leaving because revives got depleted and there is no reason to continue.
Something that can happen even in T8.
What you would be getting is a watered down M+, people will go for meta comps, blame others for failure and leave when revives get depleted.
Also the increased rewards would bring even more people from the M+ crowd since it would give another source of hero track gear, and with myth gear from bounty it would probably make M+ obsolete, since you can only get myth track from vault doing mythics.
This would in turn cause even more toxicity in delve content, which would most likely completely ruin it for you.
This is a legit issue, the crest drop from delves is like Brann yelling “found a bit of crests” instead of gold, you barely notice you got any, it probably wouldn’t hurt anyone to actually up that by quite a bit.
Something decent, like if you are required to fill the vault with 8 runs, it would be nice to cap the weekly amount in similar number of runs.