Why do you bring this up? It seems a weird hypothesis to bring up given that the parent comment did not suggest the possibility...
It seems plausible to me that the immune system might be calorie intensive to be on full alert all the time. However, I suspect that having the immune system be more active will likely lead to other complications such as autoimmune disorders or even something as common as hayfever.
That is unless of course like bats, this was the result of evolution and natural selection. But bolting on like this vaccine do it, yea, going to be pretty bad.
I like the term ancestral defaults and indeed, we've come a long way since then and our biological and environmental reality is substantially different.
There is this book series Mortal Coil by Emily Suvada which imagines a future where technology has advanced enough to allow one to tweak their genome as easily as we use apps on our phone today. It was a fascinating read.
As true as it might be, that does not mean that it is possible to work around evolution to change ourselves to fit better with the new reality.
Consequently, you wouldn't necessarily expect ancestral "defaults" to be optimal for modern environments.