Increasing the shard level cap would not solve anything really, it would mostly make the hardmode citadel raid obsolete.
Say they increase the shard cap, then you suddenly only need 5 people to get +6 gear and I highly doubt it would end up being harder or on the same difficulty level as hardmode citadel. Now there is no reason to do hardmode citadel anymore other than fellowship PvE score and for fun.
The way it currently is, high level shards are not obsolete at all, just because you can get better gear in hardmode doesn’t mean shards are not appealing. Raid armor you can’t grind for, you get 1 chance and 1 token per armor piece/weapon a week then you have to wait for next week. shard armor you can get a full set of in a day if you do enough shards.
In order to do hardmode you have to have done a lot of shards to be good enough and have good enough gear. Hardmode citadel isn’t something you just do on a whim, shards are, they’re a daily activity and raids are a more organized and less frequent activity. Because of this shards will stay relevant even tho you can’t get the best gear from them, they’re still a necessary part of the journey to get the best gear. While if you could get the same level gear from shards then the hardmode citadel suddenly becomes unnecessary to get the best gear.
Ideally you would need to do a combination of both hardmode citadel and shards to get the best gear in the game. However as long as that’s not the case I believe it is better to have the best gear come only from raids in order to keep them relevant and something to work towards.