I agree but there has to be some other earlier crisis that could be used to temporarily cut Europe off from the spice routes.
I'm not saying it's impossible, just saying it has to be quite big to disrupt the routes permanently.
You had three routes as it was:
- Through the Caucasus, ending in the Black Sea
- Through Persia ending in Lebanon
- Through the Red Sea ending in Alexandria
The first one was mostly Genoese, the second one was mixed and the last was exclusively Venetian.
Writing this, is there really a need to disrupt the whole routes? You only had a handful a cities doing the trading. Let's say Venice gets amazing diplomatic skills and manages to convince the Ottomans to evict the Genoese from Lebanon. Then you have to find something to close of the Black Sea. For example, steppe invasion in the Caucasus, the Ottomans making an active effort to close those ports or too much piracy.
It nows means Venice is in total monopoly over the last leg of the route. It means a lot of money is now idle and that those people want to get back their spice trade.
Who are the big maritime players at that time?
Portuguese and Spanish are in no way able to do anything as they're fighting each other. England I don't know but I don't think so and Antweerp, I would say they don't have the technology.
So we need a nation facing West with high seafaring tech. Some place like Normandy. Normandy traders were making long distance trade with Guinea quite regularly in the 2nd half of the XVth century (I don't have sources in English but I can find them in French if interested). That means they knew how to travel on the ocean. If those merchants and sailors are backed by Genoese money, they could go explore straight west. That would be quite interesting to them as they wouldn't have the whole Priester John thing going on and map making was way less advanced at that time as well. So it would be believable for them to go west.
There, they would probably find the pelts which, while not spices, are still lucrative and would give an impetus to keep exploring.