The Lollipop kernel is probably 3.x something. Maybe as “new” as 3.16, but considering Jolla themselves quit supporting 3.4 Jolla Phone, there’s a small chance that somebody is going to work on this (not to mention enabling sailjail, anbox or waydroid).
Our society does not optimize for Android working with mainline kernels, and this is what we get: working devices as landfill…
Now, if Xenial (16.04) would also get new apps is another question: probably not. Many years have passed and there’s always some building block actually requiring modern software components that make “LTS” only mean a couple of years in most software products.