Sounds good in theory, but there are a few issues:
I) senior squad cap, meaning we can't have people in u23s over a certain age to cover the first team without them counting in the 22.
2) we would need to commit to keeping a good number of u23s in order to be able to put a full XI on the pitch each game. Yes, you can supplement with u18s, but then you are either making them play twice a week as opposed to being coached, or taking them out of the u18s meaning U16s get moved up or doubled up etc etc.
3) leagues also have fixed fixture dates which risks clashes with senior midweek games, so more unavailability issues.
I saw all this play out years back when we were still in the old reserve league. I remember one week when the youth team played in Plymouth on the Saturday, then 9 of the same Xi played a reserve fixture against championship side Ipswich on the Tuesday and lost 9-0.
Similarly, midweek senior games meant very few senior players were able to feature, meaning the youngsters were very much left to fend for themselves in games. This meant the likes of an 18 year old Andy Gunn (as one example) spending games marshalling two 17 year olds and a 16 year old alongside him in a back four, rather than the perceived idea that he himself learns from playing alongside a senior player.
This is why loans and the odd friendly when the calender allows have become the preferred option