L2 is being prioritized because our L1 defences are already very good. We are a long-lived species, and our natural ability to fix errors is so good that it is hard to improve upon. Maybe the long-lived tortoises or whales can do it better, maybe. But we have "several nines of reliability" there.

OTOH our L2 isn't that good, mammals in general (with some notable exceptions such as bats, whales and naked mole rats) are prone to cancer in their older age. There probably is a lot of relatively low-hanging fruit there.

If you think about it - individual cells aren't very precious and if some of them gets FUBARed by something (a virus, radiation or chemical insult), it is better to whack it and reuse the proteins to build a new one, if possible, instead of wasting time and resources on reconstruction of a total wreck.

Which also means that some research into replenishment of stem cells is necessary - and this is, IMHO, the really underfunded part of the whole thing. We lose a lot of stem cells as we age. Maybe we don't have to.