I suspect one-make championships can suffer if the vehicles involved are relatively high value/rare. The series based around the cheaper, higher volume cars seem to do OK, but I've seen some very thin grids for the more specialised vehicle series - even my beloved Alfas haven't been generating full grids, although the ARCA series organisers have done a lot of hard work which seems to be bearing fruit in larger average grids than were the case a few years ago - of course they also changed the organisation that they raced under
As a relatively recent (the last 12 years or so) convert to circuit racing, having been involved in stage rallying from the mid-70s I've largely raced with CSCC and although they have done an excellent job of maintaining and managing series which provide good racing for a wide range of vehicles, it has been noticeable that even with series that use "success penalties" to mitigate against big wallets dominating, there does seem to have been a continuing trend for the amount of money involved to escalate - you only have to look at the size of the motorhomes in the paddock
I think the issues mentioned that militate against the real amateur competing are similar in racing and rallying - the base costs of time-lifed safety kit and its maintenance, on top of the licencing and medical costs do tend to prevent, or at least put off, the "casual" competitor who might want to do the 3 or 4 "local" events, and whose entries can top up a series's core competitors to ensure reasonably full grids......
All a fair way of the question of the viability of AMOC, I accept