Author Existence Failure: Ken Ishikawa. This left Getter Robo Āḥ, the last series chronologically, on a very annoying cliffhanger since the magazine it ran in was cancelled and he was unable to pick it up again before his death.
Possibly not. The Getter Robo Saga compilation manga (made while Ishikawa was alive) supposedly has a "revised ending for Getter Robo Āḥ", but few English-speaking fans have been able to read it.
The revised ending glosses over the previous cliffhanger and presents an even worse one. Arc makes it back to Earth somehow, Kamui pulls a coup of the Dinosaur Empire and develops the anti-Getter weapon "Bug", and beats the crap out of Arc. Then Getter Dragon appears and it just stops there.
Incidentally, Kei (Armageddon) is voiced by Allenby's seiyuu (Narumi Hidaka). Looks like this is one show where her crush on Domon gets reprocipated.
Super Robot Wars: Better to list which games doesn't include Getter in it, which are J, K, L and UX. Special mention in that it forms one of the corners of the "Holy Trinity" of the series, the others being Gundam and Mazinger.)
Also, most of the time it appears, it's Getter Robo G. Shin Getter vs Neo Getter, appears in Reversal and GC/XO, Armageddon appeared in Destiny, and New Getter appeared in NEO. Armageddon, however, made a triumphant return in Super Robot Wars Z2.
Shin Getter is also part of a group called the Three Gods by much of the fandom, the other two being Genesic GaoGaiGar and Mazinkaiser. In-game and in promotional art◊, those three robots are considered pretty much the biggest heroes of the Super Robot type and are always shown front-and-center.
What Could Have Been: Besides Ishikawa's death, Imagawa was the one who directed the first 3 episodes of Armageddon. Everything was shaping up for an intense mystery martial arts mecha mashup with all of Ishikawa's series joining the fray. Then he got sacked, simply for taking a little too long. He took the scripts with him, leaving Armageddon the aborted mess it was post-Ep3.
Considering what we still got in the end was still awesome, it could've been worse. Still one has to wonder...