The suit may be a one-off, tailored to fit you perfectly, but the techniques used to make it are not unique. They’ve been around for centuries. If you need to adjust the fit later, another tailor can do it for you easily.
Similarly, you can have a house built entirely to your own specification that any competent builder can re-configure for you later.
Bespoke is sometimes seen as a risky option, especially for software. But it depends where the ‘bespokeness’ is.
If it’s in the techniques or materials used then you need to worry, because only a few people (perhaps only one person) can adjust it for you later.
If however, your developer uses common, easily available, open source libraries, in a common, open-source language, on a common, open-source platform that thousands of software developers use, then the only thing to insist on is that if need be, you get the code. That way, any competent developer can adjust it for you.
Off-the-peg can seem like a safer option, until you discover that it doesn’t quite fit your needs. So you either customise it (which may invalidate the support from the provider, and/or put you in hock to the customiser), or worse, you change your business processes to work around it.
When I first started in computing, almost everything was bespoke from the ground up. Now its much more like tailoring or building.
Much better to invest in bespoke and adaptable, than off-the-peg and unwearable.