There will be some BTL landlords that have no option but to sell up, all dependant on circumstances (rent income insecurity, mortgage term, employment status etc) and those properties will be snapped up by cash buyers that want to further grow their portfolios, these are often the worst type of landlord for fixing stuff and keeping places habitable in my experience as they are in it purely for profit.
No regular buyers would be able to get mortgages in that market so the only winners in a crash scenario is cash buyers, i.e. the rich, and they will then watch as the market recovers and their 20 new 2 up/2 downs increase 50% over the recovery period all the while they will be rinsing tenants for "market" rate rent even though they have no mortgage to pay. Honestly, a crash would be horrific for 'normal' people.
Also, I feel it worth mentioning that not all BTL landlords are the ultra rich multi-property unscrupulous types, my last one in particular was what I would say is a great example - he would buy plots or run down places, redevelop them with quality housing and charge what I would call a fair rent for the area (have to recoup costs somehow) and without someone like that I would have had to settle for some of the awful properties that were in the area I live in now - the ones that someone has bought for a cheap price and done nothing to bring up to standards or modernise, expensive to heat, mould problems, rough areas etc.
I'm out of the renting game now though, although it wasn't my choice nor my desire to own (mortgage) a home it was all my partners desire and I've come along for the ride as scary as it is - on my own I was too lazy to do the footwork/saving/sacrifice to get the mortgage although as a result we are now VERY lean with minimal direct debits or unnecessary purchases or services, the lockdowns have made saving even easier as there are more limited ways to waste money on - but then that links into the economy failing so everything has unintended consequences.
