I also feel the loss, but in my opinion the entry-level Mac Pro was already unaffordable to most hobbyists. A $300 computer case and motherboard it was not. Apple never made an xMac, which sounds a lot like it would have been the thing you're describing.
Well, Macs in general are unaffordable for many people - but the $3000 entry point wasn't completely ridiculous, especially if you already had a decent display. That's really not an excuse for doubling it - especially with an entry model that really only makes sense as a base for another $5-$10K worth of expansion.
Also, I think the "xMac" comparisons are outdated - when the "xMac" concept was first mooted, the go-to system for the mass market was still a commodity mini-tower PC. The fallacy was that Apple could compete with bargain-basement PCs and still make enough money to justify investment in the MacOS ecosystem. Those beige box PCs were/are virtually loss-leaders - if you bought an extended warranty, RAM upgrade and a $30 printer cable with that, then the seller might actually have made a profit. In the corporate world, the money was probably in support contracts. So, Apple couldn't have matched the price of cheap PCs anyway but what they would have done is decimated sales of their laptops, iMacs, SFFs and (at the time) tower workstations.
Now, time has moved on - what the mass market wants mainly is laptops, if not tablets and phones. Apart from the ultra-cheap $300 PC end of the market - to which Apple's answer is now the iPad - its only really enthusiasts, gamers and pros who want towers. I think those people would happily pay a reasonable premium (in the price range of the higher-spec iMac models) for a customisable/expandable tower that could officially run MacOS and, in 2019, I don't think that would necessarily cannibalise domestic/business sales of iMacs the way it would have done in 2005.
...and the development costs of a midrange Mac tower needn't be huge. If a hackintosher can build a decent machine from standard parts on their kitchen table, Apple can certainly design a nicer-than-average mini tower for a fraction of the investment in something like an iPad or MacBook - the job is to sit unobtrusively under the desk and blow cool air over the parts, folks - it doesn't have to be a work of art like the new MP.
Anyway, its a pretty sad state of affairs if Apple is only worried about competing with Apple - and one of the trends with Macs recently - especially the Mac Pro - is a complete lack of effort to grow the Mac market beyond existing users.