I read your response, but historically each company has taken different (but consistent) actions on parts.
So Bachmann operates differently than USAT traditionally. LGB also, and also changed since purchase by Marklin.
So, I can agree somewhat on unpredictable, but in the main, over the last 20 years, USAT has been very good on stocking wear parts, much better and more consistent than Bachmann, and also stocked in the USA.
So to the specific question by the OP, normal wear parts I would not worry with USAT, they always stock parts, and it's clearly part of their profit window, meaning they have found a way to keep parts profitable, so for 20 years have been good. Bachmann and MLGB are different and have different track records.
Yeah, each manufacturer / distributor (as Bachmann is not a manufacturer nor is USAT) seems to have a different philosophy.
Aristo was the worst, the president of the company came on his forum several times and said he had no desire to have parts "just sitting on shelves", and other than couplers and wheels, only ordered parts at time of a major manufacturing run. Often when you got parts, they were cannabilized from new locos or cars, I know this for a fact from the person (Navin) do did this, and is a very honest person, who unfortunately had to be careful how much "truth" he told.
When I called the original "Mike in parts" at USAT, and timidly asked for a brake wheel for a boxcar, I almost passed out when he said "sure, what color". I can only remember a short time when the axle sets for locos were not fully stocked.
20 years of my experience, which parallels my friends of the same or greater longevity.