Man you sound very very professional, i like your reply this is the most neatest reply i have ever read in a long long time here on the Forums! (I agree to this statement and it makes sense not to keep something that will be a burden to them in the End)
My take would be to reintroduce the Game Engine in a future version of blender something that will just be a bonus like the Interactive Mode they were talking about a few years ago at a Blender Conference!
Something that wont butcher the purpose of blenders intended goal, if blender’s goal is to remain the best modeler then so shall it be, interactivity can come in Blender 2.90; i wouldn’t mind.
By then the Engine and Blender Software would have amalgamated into something more than what it is today.
You’re right in saying that @lopas and i humbly agree to your statement, but you see … i think that UPBGE is doing that for the sake of retaining its large populated user base, just think about how many projects have been stopped after the announcement that the BGE will no longer make it in to newer builds of Blender? Or let alone the fact that the BGE didin’t make it in to the 1st Build of 2.80+?
Many projects either got shelved or migrated to GODOT, look at where those projects are today!
Personally, i see no completion at all (NO MAN HAS EVER REACHED THE END OF DEVELOPMENT OF THEIR MIGRATED BGE PROJECT IN GODOT!) people are struggling alot and also the large user base that BGE had has been reduced to ash and now BGE means nothing to newbies wanting to go in to Game Design. (People Treat it as a JeRK off Engine that makes Crappy looking ps1 graphic Level Games not even games unfinished tech demos even 
)
The fact that UPBGE exists is the reason why BGE did exist and still does today in older builds and is still being used by passionate artists and dev’s, one would think why wouldn’t BF consider returning something that could be a bonus to the Package?
for example: im saying a possibility that an Interactive Mode could make it in a 2.90 build chances are High that it could be a possibility, but judging from the current interests of today it is mostly highly unlikely that that would happen so quickly!
UPBGE - And the Future of the Game Engine Integrated in the Blender Software!
My take on UPBGE is that it will bridge the Gap between Game Creation and Blender modeling still keeping that unification in 1 package though the future and newer builds of Blender as those continue to adopt new ways and new directions of doing things, UPBGE will adopt the same, but it will still allow for the creation of Games on a software that truly unified everything and presented a Workflow that will never ever be surpassed by any other Commercial Game Engine in the Market despite the fact that this is free and open source!
personally, im at peace with the switch since iam making my game but not only that, i can testify to that statement by saying that my workflow hasn’t been shaken since the start of my most recent project TBW .
So yeah UPBGE still provides me with a Healthy territory environment to experiment and to test my skills and knowledge when it comes to Creating a Game.
And in the process my experience is growing and my confidence is booming and i get to create great content, inevitably the goal is not to please or boast or anything; for me in making such a game implies a point being made out there to BF, and i believe that the UPBGE user base needs to grow that way should the BF introduce the interactive engine inside of blender the masses will flock over without a doubt.
Mainly because everyone want’s something that is well unified and has 1 uninterrupted workflow, Where there are thousands of resources and tuts for BGE and i believe none of that should be thrown down the drain at all whatsoever…
The Games that have been made on this Engine for the Past Decade are quite impressive to tell you the truth despite the technical and graphical limitations of its time BGE still holds up and stands up for what it is today and what it once was.
UPBGE will continue to carry the greatness of what BGE had and as for SaBGE i believe its a Bonus that will enable developers to continue the great legacy that BGE had in the past. (Afterall its not about the Engine for the engine is merely just a tool to get work done, my take is that we’ll see what these Engines can produces and the types of games that will be made with them)
My wish is to localize everything BGE that being the WIKI, the GAMES from the past decade and now from UPBGE , Tutorials , Resources and lastly all the builds of BGE/ UPBGE that way we can continue growing our community of BGE/UPBGE users and also noob’s and newbies.
The Goal is to give the true experience to newcomers to Game Development a TRUE Feel of what it is to Create a Game From the Ground Up with TOTAL CONTROL. (And i believe that this Engine has provided that over the years and still is till this day!)
That is all i have to say for now but the topics being rolled on this thread continue to stir the vision despite peoples opinions. (All what i have said here is Fact not my own made up feeling based opinion!)
Fred/K.S