Without mining rewards, would miners keep the network alive with just transaction fees, or would Bitcoin collapse?
If there is no mining reward, all miners will stop mining and no one will be able to process bitcoin transaction. But this is what that can not be possible. Bitcoin mining will continue and there is nothing that can stop it.
Tomorrow in this context is very near, and it is not any day in 2140 and after that, so Bitcoin miners will get both Bitcoin block subsidy (reward) and Bitcoin transaction fee that are all good for their mining business and its positive revenue.
Even after 2140, bitcoin mining will continue, block reward will continue but just that it will be transaction fees and no newly mined bitcoin in the reward.