Without mining rewards, would miners keep the network alive with just transaction fees, or would Bitcoin collapse?
theoretically yes if the demand for bitcoin remains and there’s a lot of transactions waiting then yes miners will still be profitable just from transaction fees alone and if miners are happy well then the security remains strong
I think that interest will always be there for BTC (because it's obvious it's the one to get the most of it), and such a scenario is a speculation anyway if not going forward into the many decades ahead.