You need the blood of a DA to turn yourself into a DA. You need to go out of your way to obtain blood of a DA to become one yourself, which is dangerous and will attract the wrong attention. And that's not going to solve the energy problem as DAs are beings that need to eat other people to survive to begin with.
Technically, given the circumstances of Shirley, and the description of Nrvnqsr as "one who became a Dead Apostle through magecraft," it isn't really certain that the blood of a Dead Apostle is absolutely required.
To elaborate on this, it seems that Dead Apostle is more of a "duck-type", a label attached to anyone who satisfies particular traits (walks like a duck and quacks like a duck, as it were.)
Or rather, a large class of attempts to make yourself immortal in the Nasuverse all end up with the same sort of downside: you sorta-kinda made it work, but now the natural order rejects you, so the sun burns and you need to consume other living entities to survive. (There's presumably also more to it than that, or all obligate carnivores would qualify for the last point, but... well, I think it's clear that when a DA is consuming other entities, they're consuming more than just their energy.)
Just a question about the servant stats.
So, if the master is the one providing the servants magical energy, what does the Mana stat even effect? Is it the total magical energy they can contain, or what? I've been a fan of this franchise for a while, and this is one of the only more basic concepts that still confuses me. Strength, Endurance, Agility and NP make sense, and Luck as well, for as much as luck can, but Mana?
It seems like that isn't actually true in all cases -- or at least, I've heard that Saber can in fact actually get her magical core started?
In general, note that Saber, in particular, actually does throw out
orders of magnitude more energy than Shirou can provide. It seems like Servants are catalytic in some sense; you have to front some of the bill, but the output you get is way, way more than you put in.
Somewhat relatedly, note that the thing that keeps Servants in the world isn't just prana, it's having an anchor. You need an anchor
as well as prana, so having a high Mana stat won't save you from vanishing when your Master dies unless you also have Independent Action.