John Lennon's murder was a big deal in the rock and roll world. It didn't mean that rock and roll stopped being produced.
But the Beatles did... and so did Lennon's music. More importantly, if one were looking at the change of rock over time, Lennon's death would make a great marker for the end of the dominance of 1960's and 1970's styles of British Rock and the ascendence of harder Heavy and Punk styles ...
The point is not that the OSR playstyle is over, or one can't play an OSR game - much like many bands are still influenced by or even ape the British Invasion Sound - only that the OSR as a coherent scene is gone. At one time the folks wanting to discover and replicate the style of play from Gygax's basement (or whatever) were in contact with and sharing ideas with the folks designing ultralights. This is no longer the case. The scene came out of that, and the shared ethos was "the OSR".
There are plenty of influences on that, plenty of ways that the OSR scene/style/movement evolved until it wasn't one OSR anymore. The end of G+ is simply a key event and convenient marker - like Lennon's Death. Pick one you like better if it doesn't suit you.
Everything that lives -- and I would say an artistic/cultural movement qualifies in this case -- changes. That's one of the defining characteristics of something being alive.
Deciding that something is dead because it's changed in a way that you don't like -- "my beloved child is dead and has been replaced with this obnoxious, smelly teenager" -- perhaps due to a catastrophic outside force like the death of G+, is a pretty weak argument.
I'm not fretting over a lost time. Rather the opposite - I'm suggesting embracing change and growth. The OSR has had babies, or undergone speciation... there are plenty of "beloved children" around related to older games or the play style that the OSR created, but they are distinct and varied. They even fight a lot.
I think acknowledging that we are in a Post-OSR era and that there are multiple offshoots of the original scene, movement and play style is helpful. For example, note how often people insist that the OSR should be defined solely by compatibility with early D&D editions, or that it's about rediscovering a style or methods of play used by Gygax and friends/enemies in the 1970's/1980's. By acknowledging that this was a (not THE) goal of some part of the OSR during some parts of its 15 year history, and that some Post-OSR scenes have elevated it to their primary goal we don't have to fight with them about if ITO is "OSR". Everything produced in the OSR spaces of various blogs, forums and G+ until (and again this is a somewhat arbitrary date) 2020 is OSR. Everything after that stems from it is Post-OSR of some variety or another.