I would just add that technology changes faster for some types of components than it does for others. A reference grade headphone amp from ten years ago is very likely to compare favorably to current models…and that will probably still be true ten years from now as long as it is well cared for.
The same does not apply to DACs since the technology changes more rapidly. I tell people to think of a DAC like they would a laptop or (if you are into photography) a digital SLR camera body. Even a top quality DAC from five years ago will be significantly behind current models in resolving power, supported formats and features. You would not buy a five year old laptop for critical work…so don’t buy a five year old DAC for critical listening…or expect the DAC that you are using today to still be cutting edge a few years from now.
Transports are no better than DACs (possibly worse) since many are nothing more than small board computers these days. Even the so called bridge cards that you can add to certain DACs to make them “network enabled” are actually just a small Linux computer (or similar) running on a card.
When you’re optimizing for performance + quality per dollar spent over a given period of time, it makes sense to invest more heavily in the components that are likely to give you the longest runtime: headphones and amps. Spend less of your budget on components that will become obsolete more quickly: DACs and transports.
With the all-in-one route, lifespan of the entire unit is limited to the shortest lifespan among all internal parts. Upgrading the DAC might mean discarding perfectly good amp circuitry, for example.
An inexpensive all-in-one system is fine for getting started–knowing that you’ll eventually sell it or give it away…but I personally would not spend thousands of dollars on one.