Which combination of components (Streamer, DAC, Preamp, Amp, etc.) are most future-proof?

It’s not an easy question to answer because a lot of the factors conflate. As you allude, separates offer greatest flexibility. All in ones the greatest convenience but with compromises.

Dacs/streamers are advancing at more rapid pace than amps/preamps, which are arguably advancing at slightly more rapid pace than speakers, although each of the latter two groups much slower moving than dacs/streamers/servers etc.

So where does that leave you? Well since speakers have biggest impact on sound quality and least likely to go obsolete, can’t go wrong loading heavy on speakers.

Next, sounds like an integrated amp would suit you based on your description. Tough thing with this is more and more they include dacs (the trend), which runs against your wish re portion becoming outdated. I guess manufacturers are finding consumers are valuing the convenience over the fear of subset obsolescence.

Then as to the sources, the dacs/streamers/servers advancing rapidly but the improvements are also substantial, so that’s a good thing. We want progress and improvement right?

Good luck, not sure if this is helpful but clear from your post you’re smart enough to figure out a fit that’ll work for you.

Fwiw, in my reference system I spent 45% on speakers, 32% on an integrated amp without a dac, 12% on a streamer/dac, and 11% on cables/conditioner/isolation, after considering many of the factors you mention. 

In my secondary systems, to this point the convenience, value and relatively decent quality of all-in-one type systems has so far carried the day, but I’m likely to move to something closer to my main system percentage breakdown for system 2 within next 9 months 

@kren006 Thanks for a great reply. That does help sort out the factors a bit more for me, and the percentage breakdown and the issue of "primary" vs. "secondary" system is also quite useful.

There are couple "in the weeds" factors which I need to research further. One is related to the media which may have more sticking power than we initially assume (because we’re swamped by the churn). I see vinyl resurging -- and even read an article on cassette tapes as still (somewhat) viable (more skeptical on that one, given the magnetic adhesion factor). Another is the sticking power for the delivery technology. As I watch standards for USB, or wifi routers, or codecs change, I get flashbacks of the "betamax" vs. "vhs" thing. I just recycled a bunch of "firewire" cords and have numerous old hard drives sitting in my closet. All that stuff seems more outdated than the CDs I own, despite the fact that they’re all about the same age, more or less.

Meanwhile, my AR 48s from 1983 are great. My Adcom separates from 1993 still make music. And I still have a Thorens TD166 from the 1980s, too. Those all are just examples of the point you made. I want to improve on them, but they’re not *forcing* me to with the same necessity that some other elements are.

The last factor is about distinguishing among my own motivations. I'm quite ok with swapping out and improving gear because this is hobby and about learning through experimentation. (I mean, who doesn't like to try new restaurants or new kinds of food?) But there's an aspect to this area which leads to upgrade-itis, and that's part-and-parcel of the consumerist culture that's environmentally quite destructive and not related to music listening, which has to be the main goal. I want to know why I'm doing all this.

Yep, that’s why if at all possible it is best to audition as many things as possible before purchasing. Serves two purposes: 1) offers better chance that you’ll make more informed decision according to your unique sonic preferences based on comparative results; and 2) offers some comfort that you’ve already experienced what’s out there so less ’upgraditis’ pull, I’ve found ...

Let me illustrate via a story. I live just outside of New Orleans. We have to deal with local flooding and pooling in front of our home, often. 2 feet of water! On the street! Unless you have 'lifted' your Jeep or SUV, vehicles need to be up off the street. 

The reason: the short intake pipes on the street are much, much larger in diameter than the lengthy out-take drainage pipes leading to the drainage canal. Clever engineering, right. : )

Think of the flooding in front of my home as equivalent to the sonic results closest to your ears. Speakers are what your ears are closest to. The drain opening into that canal is distant, but it sure has a bearing and impact on what you experience in front of the home.

Kren, along with so many on the forum, essentially provides the same clever "engineering" solution. 

The front end matters. Go out and listen to a well implemented front end with basic speakers. Then listen to a basic front end with "high end" speakers. 

I can share example after example of systems where the money is in the speakers yet everything in front can't and does not deliver what those speakers are capable of. It's a mistake so many love making and are proud of.

My general advice: take care of the source... in your case source through preamp. There is no need to shy away from or be concerned about an all in one front end (Streamer+DAC+Preamp). 

I'm currently evaluating a Streamer-DAC-Preamp and it is showing up my separates, with ease. : )