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.

Depends on what mode you’re playing.

Primarily, there’s casual matchmaking, which actually is full of new players. Sadly, especially in off-hours, there’s also a lot of bots hosted by some mentally unstable script kiddies (no joke, they buy in a bot hosting framework and just pop that on a VPS somewhere) dedicated to ruining the game for everyone with aimbots, mic and chat spam etc. They abuse the fact that Valve hasn’t updated the anticheat in what feels like years. Regardless, there are still tons of healthy games to be found and bots usually are kicked within seconds.

Additionally, there are community servers which, depending on the actual community playing there, can have bonkers veterans or absolute noobs. These servers often are modded, too.