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"bloke's Rule" must be read in the constitutional context of, what is essentially, "...[to be sold] if there is another "bloke" exemplar that is being played...." In this context, I would submit that the reason the trombone won't be sold is that there is not another "bloke" trombone, as there are "bloke" tubas of various keys and configurations, and excludes instruments taken out to check condition, efficacy of repairs, etc., as they are not "bloke" instruments and therefore the money to be made from them is in the context of sales and service.

I have guitars, acoustic and electric, and electric basses the same way. Solution: I always make sure I play at least one gig per year where I use any particular instrument in question on at least one song.

Remember, in my "former life," my day job was the interpretation and application of "rules."

Tubas are more expensive than trombones...

That must by why so many tuba players cannot afford shoes!!

Def that, and I’m not sure sandals are much better...

If it ain't bloke, don't fix it.