I highly doubt this scenario, because Murdy and Co may not have even talked to Amazon/MGM about Fallout, but went straight to Bethesda, just like they worked directly with Naughty Dog for TLOU (and that's if Fallout is even happening, which isn't guaranteed).
You say that but--I feel like that's ignoring the absolute *numbers* that the Fallout TV show had. If I recall correctly, the Amazon show has now become the highest viewed show from Amazon's Studios and that I would imagine they would've noticed that when beginning to formulate and research early on into the development cycle of this year.
Still would have to work with Bethesda, true, but Fallout for all intents and purposes if I am not mistaken; seems to be Amazon's big "Stranger Things" show. A show that has worldwide acclaim and has a established point that can work well for a house (and true it isn't guaranteed, but I do think it makes sense that they might have reason to easily pursue it).
You could say that with any MGM/Amazon IP. Oh we're possibly getting Fallout this year, so I guess that means Killer Klowns is coming back? Oh maybe we'll get a RoboCop house. Ah well we could get Silence of the Lambs that's been rumored for awhile. Wait no, how about Carrie? I mean we did get The Shining in 2017. Actually no. I think we're getting a Terminator house. Source: my imagination. This whole Poltergeist rumor has just been pulled out of nowhere and your reasoning/"speculation" can be applied to any Amazon/MGM horror related ip. It's silly.
I feel like I have to say that my line of deduction comes from the idea of "Probable Causality".
Just because I bring up Amazon Studios (which is the merged Amazon/MGM), it doesn't mean I inheritedly mean anything and everything is on the table. That'd be ridiculous of me to consider and think of in the sense of it. Poltergeist, for intents and purposes; is a IP that if you recall didn't really seem to get the package that Orlando got in the care and attention that was placed onto it.
Be if it was as a result of the direction taken (As
@Freak brought up), budget cuts done to the house in light of keeping others afloat; or what-not have you, Poltergeist at Hollywood left something to be desired at the end of the day for the original incarnation. We know additionally that Amazon Studios *is* developing a new take of the material for TV, so it isn't like the IP is on sacred grounds and isn't tied up in legal crap.
I can easily been wrong, have been wrong before in the past but I think Poltergeist makes sense when considering how successful Insidious was; and how there is a question of what "will" be the exclusive IP this year.