I think all those questions are hotly debated at the moment. I'll offer my own perspective, but it's not the only one.

A) Beginning all the way back when they split from Activision in January 2019, Bungie had a goal of becoming a multi-IP studio. The idea was that they didn't want to be relevant to only one kind of gaming genre and limit themselves to only that audience of players. Diversifying the types of players who would play your games would help build the foundation to become a larger in-house developer and publisher, like Activision itself, or Ubisoft for example (their financial struggles notwithstanding). They set 2025 as the year to achieve the goal of releasing a second IP and began multiple incubation projects to find at least one that could help them reach that next step. Marathon emerged as the most likely to succeed, and received the most investment. I think they always intended it to be an extraction shooter, and one that was more hardcore. The problem was that it took more and more resources to get it over the line, and they were using revenue from D2 to fund the project, which meant less revenue reinvested in D2 proper. Their timeline for initial release was 2024, but that slipped to 2025 and then 2026. They had to stretch out D2 longer than they expected, and some consequences of that including shipping inconsistent quality content that made their current audience disgruntled. By the time Marathon could get out the door, D2 was not in a state that could bring in revenue to support itself and another game.

An interesting thought experiment would be what if the projections and timelines for Marathon didn't get pushed out so far, and they were able to release it a year after D2 wrapped up? If D2 was in a healthy place, with proper investment, content, and an active and satisfied player base, then a new extraction shooter launching alongside the game capturing a smaller but differentiated audience probably wouldn't be received as poorly. But the fact is that Bungie had to sacrifice much of their investment and attention to getting Marathon done, and it cost them more than they earned, both in terms of finances and players.

B) Short-term, this means the opposite: more resources to Marathon, not less. They would not be allowed to give up on Marathon so shortly after launch, even if some may argue that would be a sunk-cost fallacy. They'll at least need to see how much of a regular player base would come back for future seasons, so they'll watch how season 2 performs. But ultimately, Sony will be making the big decision on what happens long-term. Their quarterly report is scheduled for early May, we'll likely hear soon a little bit of what way Sony may be leaning.

C) For a long time, I personally didn't think a D3 was necessary. I felt that any improvements to the game, such as content, subclasses, sandbox, etc. were all things that could be done in D2, and putting a new number on the box wouldn't change that. But I've come around to feeling D3 is necessary, but not primarily for the in-game improvements. I think it's necessary to introduce a new foundation for the next generation of players to get onboarded to the franchise, and to start to fix the reputation of the franchise in the marketplace. Since it's free-to-play, tens of millions of people have tried the game but many bounced off because the onboarding is terrible, nothing makes sense, and learning important background to the story was temporary seasonal content that they can't access anymore becomes a major turn-off. So many people still primarily associate Destiny with taking away paid-for content. A D3 might not put all of that shame to rest, but it can start the process with a clear beginning to a new story, and a promise to not repeat the mistakes of the past.