Two things here. First on FTP and second on MTX.
On FTP, I pray that MS does not cave into the trend and offer a FTP BR type mode in Infinite. I don't mean to sound like some misconceived snob, but this is a AAA franchise for Xbox. Halo is better than doing something like this. It's already been announced Infinite will be available (or "free") to Xbox players that have Game Pass (similar to Gears 5). I am guessing they are going to focus more on gaining subscribers to GPU by dangling Infinite in front of people ("play Halo Infinite now with Game Pass Ultimate!"). Ramping up that subscriber base is far more important to Microsoft - it enhances the financial value of the Xbox franchise by adding a solid recurring revenue stream (monthly subscribers) to the business. With all the promotions and campaigns, MS is really trying to build that subscription model with its gamers. A FTP BR model of Infinite would certainly weaken the appeal of getting Infinite by signing up for GPU. I signed up for it right before Gears 5 came out. I also made sure to have like ~3 years of XBL membership paid for before doing it because I basically upgraded XBL Gold to GPU for the next 3 years for $1.
One other reason I would dread a FTP BR mode is because it would surely have a significant negative impact on the rest on the online MP experience. It's also a snub to a franchise where campaign is such a major component of the games and its community. Have you heard of or seen any of your friends online playing Fortnite's campaign? For a solid period of time people who purchase the full game will likely be in that BR mode because that is what everyone is going to be playing. Infinite deserves better than that.
On MTX, I haven't followed this as closely but it appears from others' posts here they have already confirmed they are not doing loot boxes next time around? One of the few things I did not have a problem with in Gears 5 was how they approached the "Operations" and MTX. The only thing I took issue with in that system was that some of the challenges require you to invest a significant amount of time in specific game modes that I frankly categorize as "non-core" (Horde, Escape). I am fine with in Halo having some challenges tied to Warzone/whatever an Infinite- warzone equivalent will be called.
Perhaps they use that as some sort of starting point reach a happy medium. Provide players with a comprehensive list of challenges that need to be completed over a certain period of time (e.g., 10 weeks, 3 months, etc.). Players can choose which of those challenges they want to complete (or all of them) by looking at what the rewards are for each (some heavy grinds could be required for the more desirable/coveted skins). The beauty of this type of reward system is that it keeps regular users sticking around longer. At the same time, every week (or few days, whenver) you have a store roll out new stuff for the MTX - this would include armor, skins, assassinations, etc. Gears messed this up given how infrequent it was updating the store and some of the prices they were charging for certain items. As we've seen in H5, 343 has no problem cranking out a ton of armor and skins. By regularly updating the store and offering stuff that actually looks cool (but obviously not the most BA stuff in the game, which is saved for the real grinders), you increase the selling opportunities. Combine this with the battle pass/operations style which keeps players around longer, you just increased your customer lifetime value (LTV).
I am sure there are plenty of bright minds working in the finance teams at MS/Xbox that have thought through all of this and run endless scenarios. When MS rolled out GPU and decided Gears 5 would be included, they did that for very well-calculated reasons. They knew they were basically giving away an otherwise $60 game to players who would have bought it anyway. But what they also got was a ton of new players who may not have otherwise bought the game for $60. Once the players are in, the ball is in the developer's court to find ways to monetize through MTX.
I for one do not believe MTX are the devil's work. You might call them a "necessary evil" in an evolving industry, but I really do believe with the right teams behind development you can strike a fair balance.