I’m sure they could use a smaller Li-ion battery, and perhaps the EL-10 will have that. If the EL-10 ever comes (this CR2 says later in 2023, but there was a CR2 post saying it
would come in 2021).
The big advantage to the LP-EL is the blazing fast recycle time compared to AA (the Canon is a bit faster than the Flashpoint, as well). The disadvantage is that the battery discharges over time much more than something like an eneloop AA. With my EL-5, the battery drops from 100% to out of power in about 6 weeks sitting unused. I've left eneloops in a 600EX for a year and they have plenty of charge left.
Like you, I don't use speed lights that often. Personally, I use eneloops for many things around the house (especially lighted decor for various holidays), and I probably have over 100 of them cycling through always with 10-20 fully charged and waiting to be used...and they can sit for months like that. With the EL-5, I have to remember to charge the LP-EL the day before I'll need the flash, and if I forget, I'd be SOL if I didn't have four 600EX units and charged batteries always ready to go in them.