Is there such a thing as a FOREVER DAC/streamer?

One opinion here - No. There is always something to improve and advance the state of the art. I purchased a five year old DAC that was rated A+ at the time. Now it's not even in the competition. I think it's great, but will probably upgrade in another 2-3 years. 

“At the end of the day, DAC is a digital product.”

With that outlook, don’t bother to waste your money on an external DAC. You are only adding an expense of power cord, digital cord for not much gain. If you can’t help wondering, buy a DAC with a return policy to scratch your itch :-) 

If you like the sound of that amp/dac then don't worry about better just enjoy. Also that to me is one of the downsides of the amp/dac combo you're basically stuck with that Dac or you paid for it but you're not using it.

No

Maybe, maybe not.

Forever?  Who you crappin'?

Streamers have only proliferated for 

11 years since the Aurender S10 came out.

And I would never buy one enclosed with a DAC.

Knock yourself out.

There is no such thing as a forever component… well for me there may be… I’m 70. But, in general no. But on the other hand you can buy ahead. I am willing to bet that my Aurender W20SE will out perform the contemporary BlueSound for at least 20 years. My Aurender performs at the level of my great vinyl system. So, I get incredible performance now… that will likely last past the end of my life. 

This has been proven to me over and over again since the 1980’s. You get an audiophile piece of equipment and it will outperform other stuff for a decade or more and lower end stuff for much longer. The quality of the design and materials in a high end piece of equipment just isn’t easily reproducible. With the movement away from globalization this gap is likely to get larger.