The way I see it as an experimented developer, Spark2 source code is probably totally isolated from the rest of Arturia's software code. You guess this by the look of its user interface which is very specific and not scalable, but also by the lack of common features with the V Collection instruments.
Making it evolve into a Spark3 is probably not just a matter of injecting some life.
When a software has an independent source code base, it becomes expensive to assign developers to it. You need dedicated peoples for the user interface, the sound engine, support... and they can become totally devoted to a product which doesn't necessarily sells a lot anymore. Arturia V collection instruments shows an consistent user interface and set of features with probably a great proportion of code sharing, making it possible to maintain even the oldest products but Spark2 doesn't benefit from that and needs specific maintenance.
There's also a possibility that major Spark2 developers left the company and depending on the source code state, the code retro-analysis can lead to "how can it work?" and "what does this code do?" enigmas, if not deadends. Stilll they managed to rebuild the macOS version for the M1, which means that Spark2 code is compatible with recent XCode compilers so the code is not THAT out of maintainability.
In my humble opinion, going for a Spark3 would mean AT LEAST a total re-engineering to integrate the common user interface, sound engine, MIDI management, interfaces to the DAWs, communication with other Arturia products (MIDI control center), compatibility with Spark2 features etc. Considering DrumBrute and their recent hybrid and hardware products, I'm not sure it's in Arturia's strategy - and interest - to spend time and ressources on an isolated software product like Spark.
It can also be a marketing decision not to push Spark2 anymore, in order to push Brute products and V Collection. Or maybe a new software drum machine is in development and Spark2 would be on its way. Or it's a problem with licensing some stuff which forbids Arturia to draw a new product from Spark2. A lot of non-technical issues can stop a product life.
In fact, if I were at Arturia, I would maybe consider rebuilding a Spark 3 from scratch using the common code base rather than re-engineer it, or even orient it as an hybrid product like Microfreak, taking care not to be in competition with DrumBrute though.
It's a shame that Spark2 is at end-of-life as it still rocks as a drum machine even by today's standards but products have to die some day or your product line is frozen and the competition takes advantage of it. Compare this to the cute Analog Factory software I think I bought in 2006, which ultimately became Analog Lab but wasn't maintain anymore by itself at some point so it would leave the place for the Analog Lab line.