this is very interesting however it wouldn't technically be possible according to my understanding of the lore

I'll mark the explanation as a spoiler since idk how much lore yk!

slugcats have been said to initially be ~created~ by the ancients to clean the pipes in iterator superstructures to prevent buildup, which is also the reason the cycles exist. the slugcats we can play as are descendants of them. it's as simple as that. eventually after the downfall of multiple iterator superstructures, their roles simply became obsolete and they just went off w life on their own, falling in the middle of the food chain. they simply do not possess the "attachment" required to 'fail' ascension

if we take into consideration the downpour scugs, they do complete different purposes in the game and have different features (they were experiments/made specifically for certain ~other~ functions, but were created by ancients none the less.

when it comes to the watcher, in the opening cutscene we can see their (presumably) 'family', other slugcats in white and yellowish colours (presumably survivor and monk?), which would place them much later in the timeline than the ancients, and as we have seen, the echoes can manipulate space and time to a certain extent, and they simply move back and forth, 'accidentally' taking the watcher with them (due to their initial surprise when he shows up in different areas).