Martin asked, "Still have no answers to: just how many P48 engines has Sunset brought in in the last decade? How about the other importers for that matter?"
I'm not sure I understand what difference it would make to the conversation. Having said that, the short answer would be, none. So what? Sunset did produce replacement drivers, in P:48, for some of their steam engines. If memory serves, they were for the SP/UP mikes and, later, the SP 2-10-2. Glacier Park produced conversion kits for their models and so did Overland. Were there other? I don't know, didn't keep track. Conversion sets are available for Sunset diesels.
The question about response to marketplace prompted my questions. That importers are providing or have provided at least some reasonably available parts for conversion supports your contention of growth in interest. However, that still seems spotty at best.
So what are the costs and sources of parts for converting steam engines that lack such sources?
Hey Martin! You've been playing with trains long enough to know the answer to your tongue-in-cheek question.
Steam locomotives require either drop-in driver conversions (uncommon) or a machinist to convert/make drivers (the usual way). Not a hobby shop or eBay item but something relatively uncommon and varying in price due to the skilled labor required.
Sadly Joe F seemed to carry a grudge or he'd have done fantastic P:48 conversions. No idea why, I could have as much fun with P:48 as I do with 3 rail. I just prefer P:48.
Luckily, diesels are often relatively easy to convert. With NWSL (soon to close ) offering wheels for Atlas locomotives and with Right-O-Way offering drop-ins for Sunset locomotives that isn't the big issue that it is/was for steam era modelers like me.
The lack of a "Snap track" or "Shinohara" quality ready-to-use turnout is a much larger issue in my very humble opinion. This permeates standard Ow5 since the only ready to lay turnouts or crossovers are the crappy Atlas/Lenz European profile ones. Everything else requires handlaying track. Time consuming, but it certainly IS something this scale really shines in since superdetailed O scale track looks more real than the smaller scales.
The OP, as a modern modeler, represents a growing segment in a largely stagnant hobby. If he wants P:48, so be it.