I think that there are a couple problems here,
1: every single time Crawford says something like "d&d is about letting you tell your story" he's talking about players not GMs
2: At no point have I said anything that hints at Bob being the GM rather than a player. I have no idea how you came to the conclusion that Bob is the GM making up an adventure on the fly while ignoring the adventure he himself prepared while citing "mY cHaReCtEr..." while roping his fellow players into the story he showed up to "tell".
3: Often the other players just want to show up & play d&d no matter what the adventure is & when they have no reason to be personally invested in caring that bob is trying to drag them off in search of a grue that did not exist until the GM sighed & started building an adventure on the fly for the story Bob came set to
tell through the GM.
4: If you somehow got that mixed up, your claim of
feature rather than bug can be discarded.
If they are doing everything except for "the goal" & only managing to have a session at all because the GM is building stuff on the fly instead of running prepared content that leads towards "the goal", they are doing stuff without working towards "the goal" and without experience points or some other numerical system of tracking progress it very much results in pure milestone leveling being stuck without advancement. Without advancement player frustration eventually points at the GM rather than Bob's endless sidequest chasing.
Some of this is undoing your weird fisking & is going to repeat pr rephrase things already said but fisked off as a separate thing to be questioned in isolation... They absolutely
could get benefits regardless, but without an immediate feedback leading to an immediate question like "
Hey GM, why did we get so little treasure/experience from that?" there is no immediate "
well I had to improv the whole thing because you guys are ignoring x y & z along with all the attempts I made to point you back at the adventure instead of whatever bob was chasing so I didn't feel giving more would be reasonable". Without that immediate

all eyes on bob

there is no reason for Alice Dave Cindy & Ed to tell Bob how they already have an adventure & remind him of all the in game/personal reasons they want to do it more than the story Bob came wanting to "
tell"
The GM absolutely
could say no to Bob... -BUT- the GM doesn't want to just slam the door & make it look like they are railroading their players. The other players likewise absolutely could say no to Bob -BUT- once again... frequently they just want to show up & play dnd no matter what the adventure is & Bob's
subverting of the adventure to go off as ome of his sidekicks on his Bob's llatest sidequest is as good as anything else when they themselves have no personal reason to care what adventure they go off doing.
That bold bit is not true and more than one of your "suggestions" demonstrates that 100%.
- if the players are getting experience regularly and get what they feel is bad experience doing Bob's sidequest the GM built on the fly they immediately notice that they get less than expected or nothing. With milestone progression they regularly get nothing anyways & don't bring it up until "hey gm, it's been a while" unless you are awarding milestone levels like every session or so.
- I feel like I've covered this more than once today. This is the edition that expects no gold & treasure with no real needs to spend it on if it is gained. Getting less than players feel they need or expect while going off on Bob's side quest that the GM built on the fly lacks any weight to be immediately concerned about when using milestone leveling in such an edition. Beyond that... How do you expect rewarding players for going off on bob's quest to subvert.
- Reward? Why would the players be rewarded for going off on Bob's little sidequest instead of a quest that might lead towards the next milestone goal?
- This ignores the fact that Bob doesn't care because he came to the table with a specific story he wanted to "tell" and often the rest of the group just wants to play dnd without really caring what adventuring they go off doing. Also I was talking about why milestones invite problems that are actively hindered by using some quantifiable scale of progression like exp or gold for exp. The ones I'm familiar with are just milestone awarded=+1 level... what milestone system are you referencing (maybe a book and page number?) where players track a pool of "milestone achievements" till they bank enough to level? Why would rewarding the players for going off on bob's endless sidequests while the world hypothetically burns accomplish anything of note?