I don't know about Nimoy. It took a major motion picture and Robert Wise's input (if I remember correctly) to get Paramount to settle the suit and bring Nimoy back into the fold.
I imagine that, without him, the series would probably have been lowly rated and quickly cancelled. But, of course, it's all speculation.
You're right, it's a speculation.
So here's my speculation: If it was highly rated, Nimoy would've campaigned to get back in. If it was low-rated, Paramount would've settled and tried to bring him back to jump-start the ratings.
They would've promoted the Hell out of Spock returning either way.