Again, barring radical superhumanity, in many cases companies do not today want their own bespoke solutions to a problem and wouldn't want it even if the developers were working for free, because the other costs to the business would still be more than they want to pay and more than another business can charge to make the problem more thoroughly (even if not completely) go away.
I suspect this is another reason you may see some companies making grabs at data and points-of-presence that otherwise make little or no sense... they're trying to colonize and defend the sources of contact with the real world before someone else gets there and locks them out.
With insignificant concerns, at times it seems a business might not even care about the cost but that's also the same areas where a cut in quality is ignored if it comes with a cut in costs as well.
A lot of SasS fall into this category, so even if AI is not great, the value of getting a solution in under 5 minutes, simply by talking to a machine, is too great a value for a business.
All those marketing sites that might've been handled by 3rd party agencies in the 80s, became in-house clip art projects in the 90s, became SaaS sites in the 2000s, and will become "CEO told his dreams to the AI, and the AI delivered while they were being driven to work" projects in 2030s.
The fact that it's bespoke or not is besides the point when writing up the requirements means the site is essentially already built and there's no need to vet vendor at all.
Now for significant concerns, SaaS will likely still have a place, but some aspects of it will be minimized. Companies focused on Wordpress, Jira, and Salesforce customizations will fall by the wayside since the main company can just say "talk to our AI agent and it'll customize it for you".
It doesn't say SaaS is being replaced by agents, rather that SaaS companies will be transformed in the way described. Outsourcing will still happen, hence the discussion of headless components, and the continued demand for SaaS.
And part of the argument is that we won't just be waiting for the new breed of LLM-native companies; lots of existing businesses are being LLM-ified right now, through the process described in the article...
That said, your point can be distilled down to: comparative advantage. It still exists and so SaaS will likely still exist. But per the article, it'll probably look pretty different (and a lot less profitable) over time.
I think the reason it'll look different is that it's very hard to make powerful software that's also walk-up usable. The idea that you can provide just the core concepts of a piece of software and allow AI to interact with those concepts more directly, or allow your users to use AI to build their ideal (and ever-changing) interfaces to your concepts seems pretty convincing to me.
Of course, nothing is really preventing the SaaS side from operating the agent swarm, yes?