How good is good enough?

Once you get the room right, size, soundproofing and acoustic treatments, it will be much easier to get over that upgrading equipment need. With an average room, the upgrade equipment bug will never leave you. At least it has been my experience with the house of stereo. Complicated, expensive and lengthy project but I have never regretted it. You get what you pay for is as true as anything in room acoustics.

I’ve spent enough money on upgrades that provided a barely detectable difference (note: not necessarily improvement) despite reviews and recommendations that promised otherwise. So, any future upgrades will be out of shear curiosity or gadget-interest more than an expectation of audio nirvana.

I am done.  Until I change my mind 

It stops when you find out what makes sound do what it does.  It is not pricing that dictates sound, it is design and parts quality.  You can upgrade your existing components using better parts that the manufacturer used and get fantastic results without spending that much compared to what are considered reference components.  We do this all the time probably 5 units each month.  Even $20K units are improved greatly.

For me acoustics was more important in my learnings and in sound quality that the price of capacitors or the price of crossovers ...

For many it is the opposite...

But it is better to have perfect room acoustic with less good capacitors and crossovers than the reverse...

Any showcase with hundred of thousand dollars system mediocre sound quality is proof of my claim ...

Most audiophiles are focussing on gear, not on acoustics as much... They conflate acoustics understanding and experiments with few panels on a wall ...