"living in private rentals, the most urgent thing is to sort out the lack of security of tenure there."
As I've said above trying to tinker with the housing market is complicated.
If you force landlords to offer longer tenancies to people the immediate effects of that would be that a lot of landlords would give up and sell up. That would reduce the number of properties available to rent which would drive up rents across the board.
Most landlords are not evil bloated cigar smoking figures in top hats. Most are "normal" people who, particularly at the moment, have trouble selling a property and decide to let it out. Most don't make ridiculous amounts of money and most don't exploit their tenants.
It's those ones who, when faced with the possibility of their property being locked into a tenancy agreement for several years rather than just one at a time, who will back out.
Leaving the bad landlords to enjoy the chance to charge even more rent than they do now.