I also think Chad was the original plan for the OST. However I don't think Marty/id ever decided that Chad isn't up to the task. Note that Mick only got the contract by going straight to their parent company Bethesda. Marty only showed up after Bethesda was already doing the contract anyway.
What's more, the announcement of the OST has some ambiguous wording. They announced it as Mick Gordon's DOOM (2016) and DOOM Eternal original soundtracks. This stuck out to me immediately as a combination of Mick Gordon's DOOM (2016) and DOOM Eternal. Mick cites this in his medium post as Mick Gordon’s original DOOM Eternal soundtrack but I think that is (understandable) wishful thinking.
What's more, the announcement of the OST has some ambiguous wording. They announced it as Mick Gordon's DOOM (2016) and DOOM Eternal original soundtracks. This stuck out to me immediately as a combination of Mick Gordon's DOOM (2016) and DOOM Eternal. Mick cites this in his medium post as Mick Gordon’s original DOOM Eternal soundtrack but I think that is (understandable) wishful thinking.