If being a landlord were outlawed or made unprofitable by significant new taxes or whatever tomorrow, most businesses that rent today would relatively quickly be able to put together loans to buy the properties. Yes, starting a new business would require getting more capital together—but since this would be a change across the board for real estate, it's hard to imagine that banks would be unwilling to provide mortgages on such properties. It might mean that slightly fewer small businesses get started that already have very little chance of success, but honestly, we need to be solving those issues by different, more systemic, means anyway.
At some point we have to stop accepting this narrative that monopolists and other rentiers are doing us all a favor by "taking on the risk" of actually owning things, freeing us of that "burden" and graciously allowing us to pay them for the privilege of that freedom...forever.