Sorry, had to highlight this one; this argument is objectively false. It has been brought up regularly with miniature games and edition changes; I've seen it for Warmahordes v1 to 2, then 2 to 3, Malifaux 1 to 2, and WFB to AoS. I could say it about Relic Knights too, but someone would've had to play Relic Knights.
The cultural pressures of tourney play, new shiny models, new rule support, and comparative ease of pick up games grind out the older version every time. These games are not RPGs, where the competitive players are counteracted by the difficulty in picking up a new system, and there's not a terribly functional concept for competitive play, and pick up games aren't really a thing. You noted Necromunda, which is a really good counter example. I am curious how many people play Chainmail. Or more pertinently, Warhammer 40k 6th ed. Or Squats.
To place the onus on someone to compete against a newer edition of the same game because they're frustrated by a surprise sunset on their purchase is not cool. Please stop that one.
I disagree on 2 as a viable argument, as it essentially gets to the core of the concerns; the impression is clearly that these purchases will not be supported when the assertion is that they would be. Telling them that they might get support is kinda missing the reason they're annoyed.
Arguement 3 (Dead Hand and TO etc) are addressed elsewhere, but generally you get more people playing with accepted tourney structure and minimal adjustments. Unless there is a very compelling counter reason, Dead Hands are probably going to be exceptions.
Arguement 4 is true. Of course, so were Avatars. I can't say I've used one outside of the beta test, but I might be very wrong here. What have you seen?
And to the general thread, 3rd edition looks awesome, I love how the cards appear to have taken cues from TOS, and I want those faction headers as wall scrolls. I am a bit concerned with the Malifaux's app's future. I use it religiously and got more than a few friends to buy the decks. What plans does Wyrd have with that regarding 3rd ed?