Doing just payments is a really low margin industry, and the larger the contract you are after, the lower the margins. Therefore, it makes a lot of sense to try to sign up startups, as their growth is your growth. And to win in the startup market, you don't win by lowest costs, but by how much generic work you can save them. Cut their headaches, and they'll be happy giving you a wider cut.
Thus, a million little acquisitions to make the possible Stripe bundle for small companies stronger, as they become the moat. The opposite of, say, the Adyen play, when you want to lower your own costs, and make money on tiny margins to do processing for really large companies.
My analysis is that Stripe wants to own the costs of running a startup. They would get a complete picture of both sides of the business: what money comes in and where it goes.
There are also a couple of advantages. They can take money directly from revenue before it leaves Stripes and without any processing costs. They can also invest into startups through credits and financing. And finally, their exposure to bankruptcy risk can drop as well.
AI is making it a low-moat industry too. It's a lot quicker to build some API docs, a behavior tracking JavaScript library, and some fraud detection, with LLMs.