I think “how accessible…?” is probably too simplistic.
It’s easy to answer – as it need cost very little to get started, but that only part of the picture.
It’s possibly more to do with the route into MTB that people take.
eg. We recently had a new rider, armed with an aging BSO, come along on an easy Saturday club ride. During the ride various riders helped sort out her gears, her brakes (which were binding badly), a cup and cone hub (which was lose), etc. So yes it’s very “accessible” to start with.
… but then other factors kick in…
The bikes that most of those nice, helpful, seeming knowledgable folk are riding are typically going to be around the £2k mark.
Despite her mechanical trials and tribulations, she had a whale of a time and was hooked (yay, success) so sought advice from those same folk as to a new bike purchase.
Good advice was given (go try lots etc) and not long after, a rather nice new F/S trail bike appeared.
So, my angle on this is, yes it’s accessible, but soon after there is an effect based on experiences/norms, what other people have/advise/etc.
This is what might appear not-quite-so-accessible.