Having watched TFA and TLJ a few times, I noticed that the only non-BB astromech droid we see is R2-D2, and he spends most of both of those movies in low-power mode and looking fairly worn out.
Are R-series astromech droids obsolete technology by the time of TFA? Is there any other explanation for why we see many R-series droids in the first six movies, but only BB units after that? As of TFA, R2-D2 is at least 65 years old (as he was fully operational in TPM, which was 65 years before TFA), but that doesn't fully explain why there would not be more recently built R-units around, or why a 65-year-old droid would necessarily be considered obsolete.
Do we simply see no other R-series droid because they are truly considered obsolete tech that no one, Resistance, First Order, or other, would use unless they had no other choice (or were guided by sentimental reasons), or is this simply an artifact of the fact that the story is concentrated among small groups of people and places and the absence of an otherwise common R-unit is simply by random chance?