From my understanding, it wasn't so much a falling out where things became unprofessional or personal, but rather a difference in philosophy over how the club's roster should be shaped over the next 6-7 years versus the desire to strike while the premiership window is open over the next 2–3 years.
POS was staunch in his belief that the club needed to limit spending outside the spine and assign a maximum value to positions and players based on data, forecasting and long-term projections. The goal was to prioritise internal development and astute recruitment to replace any players lost along the way. This obviously leads to some tough conversations and can leave players feeling overly commodified during contract negotiations.
Holbrook and Parr are much more people-oriented, empathetic and relationship-driven men who are more inclined to factor those qualities into negotiations, whereas POS is well known for leaving them off the table when determining contract value. I think Best mentioning that he's a father now alludes to something that POS simply doesn't see as relevant when putting a dollar figure on a player.
It basically comes down to this: POS's philosophy is that if you consistently compete for the top four over the course of a decade, you'll eventually jag 1–2 premierships.
Holbrook and Parr, on the other hand, believe that keeping the current squad intact is important and that winning a premiership or two here and now will have flow-on effects that benefit the club for the next decade, particularly in terms of player retention, recruitment and junior development.
None of this is to suggest any of the parties are wrong. Everyones got a job to do and POS is the best in the business, but there is a certain 'magic' about the current squad that is heavily driven by the emotion of being underdogs and proving they belong that flows from player to player and POS meetings were a bit of a circuit breaker to that current flowing through the place and Parr and Holbrook identified it.
I don't think this is a lets see who's right type outcome, e.g. will we overspend now and get gutted over the next 3-4 years and have no premiership to show for it and end up looking back that losing POS killed our golden window of opportunity. Or if this squad wins a premiership in the next 2 years then takes unders/cuts in the new raised cap to stay together to win 1-2 more that only losing POS made that possible. I don't think our premiership window is on that big of a spectrum - we just need to capture this magic we have now and turn that into a sustained identity and culture and premierships will come.
It's a complicated business that's for sure.