It’s a very nice question indeed, wherever SaaS working SMB-market could go up-market (Enterprise), and what parameters/early indicators may potentially affect it. I would be curious to know how would one estimate a potential to grow up-market.
However I also believe the contrary is also happening. There are some companies in pure Enterprise-sales, that are trying to cut larger product in pieces and sell it to SMBs. Selling to large number of SMBs, could potentially have a very nice effect on margins, as well as growth.