I think they don't necessarily say that Marathon will be bad (although most signs are pointing that it will be), but that Extraction shooters are very niche. They don't offer long-term progression the way MMOs do yet still require a lot of time (both per match and overall), which drives away a lot of players. Because of these limitations, Marathon wouldn't be able to recoup it's enormous development cost even if the entire extraction shooter player base moved to Marathon over night.

Now add on top that Marathon looks like absolute slop because the suits at Bungee fired everyone remotely experienced to cut costs and the result will put the game closer to Concord than to Escape from Tarkov (afaik the current #1 extraction shooter) in terms of success.

If you are interested in (new) extraction shooters, maybe have a look at Arc Raiders. That one looks excellent.