Well, as I mentioned, I don't think it helps reduce mite populations, nor do I think it has made my colonies "healthier". It does introduce an issue with cleanliness if you put in a temporary board below the screen. If the bees can't get to an area, they can't keep it clean, it then serves as a nice reservoir for other pests (ants, spiders, mice, various maggots, earwigs, etc) and mold & mildew & moisture - and sometimes bees take up residence and get trapped there).
Really the big benefit is counting mite drops (this can still be done on solid boards, just not as easily, or you can do a sugar roll), and it does help when you want to contain your bees so they don't get baked when you totally close up a hive (when they are getting robbed, when you want to move them, when you want to contain them because farmer next door is spraying his fields). But, I can get a screened top for that and lift the tele-cover a bit.
It just takes a lot of simplicity out of beekeeping to have more complex screened bottom hives and I am not sure the bees like it.
Admittedly, in San Diego you could keep the screens open all year - well some do that here as well, but I just can't do that when we have winters like this year where it was below zero for like a month in a row - so you wouldn't have issues with contained detritus - but I am not convinced your hives will be healthier .
In regards to lifespan . . . I am not sure that 2.5 years isn't reasonable, but I am not sure what we are talking about here - the life of a colony with a given queen? Then 2.5 years is decent. You might just consider requeening a bit more often, say at 2 year or 1.5 years. Only hive that I have had that lasted 4 years without some type of queen manipulation is my observation hive (and it swarms 3 times a year at a minimum - because it is 3.5 frames in size and overcrowds quickly).
I think your idea of keeping a few more hives and making splits is the ticket (and catching swarms :))
PS - my wife was walking around downtown San Diego today and there was a swarm of bees in some major shopping area trying to find a spot to land on. She wasn't able to hang around to watch the spectacle (hard not to be fascinated by a swarm - even when you have seen dozens).