I have many thoughts on this situation, and it starts at the beginning.
Moresampler was praised as the best resampler to exist, Arpasing was praised for it's intuitive nature in comparison to English methods that came before it. SynthesizerV became a phenomenon of its own, creating a bubble that would grow larger with time. I'd say that SynthesizerV's success was at it's core built on the backs of the UTAU and Vocaloid communities, especially with Quadimension's support. This happened at a time when most fandoms, regardless of language, felt very betrayed by Yamaha. As it has been mentioned, yes there is a parasocial aspect at play, but it's also been a decade of catering to our groups. With V5, we saw Yamaha perform a complete 180 of the marketing that they had been holding since the late 2000s, leading what arguably caused the vocal synthesis boom (Hatsune Miku) leaving the brand [that's a separate discussion].
This was a huge blow, especially since almost every major company jumped ship with it or simply ceased updates. For an already slim market, English fans had only one alternative (and arguably Chinese fans as well with how slow the progress was on Vsinger). This was the perfect time for SynthesizerV's debut, and as mentioned it launched much higher than expected due to the perfect culmination of these circumstances. I definitely agree SynthV was viewed as a "savior" for the community at large as most felt Yamaha was completely tired of Vocaloid.
The problem arises when that savior is...not sufficient. Dreamtonics has never been good at marketing, but as the bubble increased so did the difficulty to maintain it professionally for Dreamtonics. SynthesizerV R1 had little-to-no controversy (I can't even recall any), but now that we're seeing the brand expand without people who can nurture it we're seeing the bubble burst. It's blatantly obvious that Dreamtonics has no person, or a very inept person, in charge of marketing and PR. For a company and brand that built their success off of a community in the way it did, this was bound to happen sooner or later and it explains why it never caught on in Japan. Japan never felt the betrayal in the same way that China and the English speaking communities did, why would they? They still have Miku, they received the Meika twins, and they've always been very well fed with content and have more options than any other commercially supported languages combined.
Through this latest controversy, I would say the SynthV bubble has finally popped; like it already has in China (or at least died down). People are just fed up, and I think rightfully so seeing as how there still hasn't been an official statement regarding this situation (of which just happened to also be poorly timed with the Diff-SVC attention).
Dreamtonics is not an indie brand, they have the budget needed to be successful, but instead of doing what is owed to their business partners by properly maintaining their brand and product, they're just not even bothering and there's many cracks showing. DT publishes updates and new features without proper optimization, they don't engage or address anything with consumers (and the once or twice they have have been a nightmare), as well as overall having a poor quality of service that isn't their tech. They need to get their act together.