Didn't say it was a walled garden. But management has its own ways and quirks I said it was possible that the situation was seen by mgmt as a walled garden.
And I answered to that already on my second paragraph.
Taking the nuclear option after merely "seeing [something] as" risky without exhausting the much-cheaper remaining options is not "somewhat understandable, if not plain reasonable". And it's not "ways and quirks": it's incompetence at best or corruption at worst.
This kind of situation might be common, but it is not understandable nor reasonable.
For better or worse there are tons of both reasonable and unreasonable factors as to why a large company would replace a part time developer's side project with something that costs 9 figures.
You don't know those reasons, the person you replied to doesn't know those reasons, and in fact the OP probably doesn't even know those reasons (they "used to turn up to that customer annually for maintenance").
Without understanding that it can be simply and cheaply fixed by training second person is gross incompetence. Those single cell morons should've been fired instead.
Taking the nuclear option after merely "seeing [something] as" risky without exhausting the much-cheaper remaining options is not "somewhat understandable, if not plain reasonable". And it's not "ways and quirks": it's incompetence at best or corruption at worst.
This kind of situation might be common, but it is not understandable nor reasonable.