Then maybe an adjustment to the ascension perks are in order. Perhaps cyborgs should be the pinnacle of their own branch, instead of just being synthetic ascension part 1. Pure synthetic ascension could still offer unique perks too: just to spitball, maybe your pops have the ability to colonize barren worlds, with complex space-code that ensures absolute loyalty (don't want any of those pops migrating away or being conquered, since then your foes could use them to conquer barren worlds too, so they never migrate away and if they ever get conquered they shut down or something. Maybe even the second part is only if you also have Enigmatic Engineering?).
I'm not sure what the answer is exactly but "balance" is a deeply dissatisfying answer to why something should be impossible in game.
Sort of like asking to start with Psychic pops, or with Erudite+Robust pops, or to start on a ringworld or ecumenopolis.
Or that could work too. We have government civics right now that cannot be removed, to represent powerful, fundamental aspects of your society. Why not have one that grants you a perk for free (you would pick in the government menu of course, not in the game, to ensure when the AI does it it is still the right one). It would also grant you the +1 perk tech for free to pay for it. In exchange, you can never remove that government perk. Or something like that.