Spahi were fine when the rest of the civ was balanced around Spahi being op. They are hard to obtain and it worked.
Unfortunately, the other parts of the civ have been buffed and the weaknesses that once kept Ottomans in check have largely been removed.
You’re incorrect about it only being Ottoman mains who oppose this. I’ve been a vocal supporter of nerfs to the Ottomans for a long while and I disagree with your suggestion here. I won’t link to too much, but I made a thread a while back: Ottoman’s Historical weaknesses have been removed