You know, if Phase II had happened, I think we'd get something akin to Buck Rogers or BAttlestar Galactica effects wise (although Galactica had the benefit of John Dykstra doing the effects for the first "movie"). Those effects weren't too bad, the episodes are actually a lot of fun to watch. I think it would have done well given the scripts I've read.

But I do think it would have had the same end as those two...no more than two seasons, off the air and considered a twice failure. TOS Trek's jump to the big screen was big enough to jumpstart Trek again, and TNG came along late enough (and with that first run syndication, meaning low ratings weren't nearly as destructive), to allow for a "TV Branch" of Trek to survive.

https://www.slashfilm.com/1659186/gene-roddenberry-star-trek-phase-ii-unmade-series-details/