Looking at the PSN user reviews comparing it to known sales of other games (considering if download only considering the average digital sales for games also available in retail), Marathon seems to have sold in PSN around 800K+. In Steam seems to be now at around 1M+. So adding other PC stores and Xbox pretty likely is around 2M or slightly above.
We have to consider that in addition to the $40 of the game there are the addons (digital deluxe editions, collector edition, battlepass, cosmetics). Like Helldivers 2, Marathon started with a limited number of addons which will be stacked (battlepasses won't expire) with the new ones. There are many cases (like Helldivers 2 or EAFC to name a couple) where the paid GaaS generate more from addons than from game purchases. So the addons are very important but still is too early to see their impact because they have to keep adding more and player have to keep playing more days to buy them.
We saw they didn't kill the game just after release and that plan to continue working on it, which means it wasn't a disaster. Pretty likely when just after releasing the season 2 (so a new batch of skins, battle pass, big batch of content and features, etc) in a couple months they'll review the status of the project and will have enough data to more properly evaluate how the game did in the launch window regarding revenue/profitability, active userbase, user retention, first average revenue per user, etc. and start to make long term projections.
Let's say by then they have sold 4M copies, that generated on average until then $55 each and with early S2 numbers they project that will make in the long term $80 per copy and Sony gets (after taxes, store costs, refunds etc) on average 75% of that. That would be around $240M around the start of season 2, having the game secured profitability (îf my $200-250M estimate is right) or almost, and future seasons get self funded adjusting resources as needed depending on performance.
Let's say my estimates are a bit too optimistic and they need a season or two more for the break even. Despite pretty likely not having achieved their pre-launch estimate, they'd still get the break even in the first year, which is better than many games, many of which never even get profitable.
Regarding the decrease, I'll post later the comparision with many of the most successful games of related GaaS subgenres. I assume that like last week won't be as bad as you think.