What I see is that typically, manufacturers come out with "new, better" products every two years or so. This to me is to keep their name in the papers and especially the magazines that review products. This is true for solid state and tube manufactures. Pass labs, Jeff Rowland, etc. Just because AR provides an "upgrade" to their products doesn't make this any different. I prefer the upgrade than a wholesale issuance of a "brand new and better" product.
The new and better product is really not, just new and different tubes, different capacitors, upgrades in the power supply, etc. But basically the same circuit design.
Also, the resale value of upgradeable products is higher than the resale value of completely discontinued products.
To me (in my humble opinion), it would appear wayyyy more disingenuous if a company came out with some new and better product and discontinued the other product after a year or so.
You have to get products out of the door eventually. To keep doing R&D forever would just be stupid in a business sense. So, some bright Engineer finds a better capacitor or better tube a year or so later. But the basic circuit design is very good. I would upgrade the existing product, instead of coming out with a new product that is basically the same.
In my opinion, the AR REF250 is hands down the best amp I have ever heard. I imagine the circuit design is the same for the REF75 and REF150. Why come out with entirely new amps? Add the simple upgrades, offer them to existing customers also, and move on.
I choose to not believe that AR would intentionally come out with a "subpar" product with the final intention of offering an upgrade later. What is the point of that? They could have just as easily come out with an "entirely" new product (that was basically the upgrade) and called it something else.
It is a great company, that puts many people to work and in direct comparisons with other "great" products, they are equal to or better. So, they are doing a great job.