Recovering enterprise SaaS PM here. I don't necessarily know that a lot of enterprise SaaS will disappear, but I do think that a lot of the companies that build it will go out of business as their customers start to build more of their internal systems with LLMs vs. buy from an existing vendor. This is probably more true at the SMB level for now than actual enterprise, both for technical and internal politics reasons, but I expect it to spread.

As a direct example from myself, I now acquire and run small e-commerce brands. When I decided to move my inventory management from Google Sheets into an actual application, I looked at vendors but ultimately just decided to build my own. My coding skills are pretty minimal, but sufficient that I was able to produce what I needed with the help of LLMs. It has the advantages of being cheaper than buying and also purpose-built to my needs.

So yeah, basically the tl;dr is that for internal tools, I believe that LLMs giving non-developers sufficient coding skills will shift the build vs. buy calculus squarely in the direction of build, with the logical follow-on effects to companies trying to sell internal tools software.