Several folks have tried to dismiss the need for slower leveling by experience by suggesting the gm just give less experience for them to just use milestone leveling to slow it. Both of those options strip the benefits of experience point leveling or create new problems though.
With an experience point system and reasonable players the gm Is able to do things like give a player an extra little reward for things they do that are especially noteworthy or deserving of merit. In a linear progression those might stack up into outsized gains, but with each level taking roughly double the one before it they mostly melt away into maybe a session or two early advancement rather than multiple levels from bonus xp. If the gm is giving out a fraction of experience that reduces how much the gm can award as bonus xp and increases the time between advancement for very (in)competent play. When the gm is awarding a variable fraction to do secret milestone under do it makes those bonus xp awards either meaningless or incredibly effective depending on how and if they are reduced against the variably reduced XP from adventuring.
Milestone advancement by comparison not only removes the bonus xp option from the GM's toolkit, it also creates a situation where the gm needs to either channel comic book guy from the Simpsons if a player is regularly behaving in ways that cause disruption§ rather than rewarding a second player who is picking up the slack to minimize disruption.
These disruptions are not simply because life unexpectedly happened, there are literally people who feel it is reasonable for a player to come to the table with an attitude of I think
"don't expect anything from me" is the default and if you want something else you need to state it." should be the
default expectation.
The impact of milestone leveling on enabling disruptive player behavior to cause disruption when an attitude of "I showed up" is the one they take doesn't stop there though because under milestone leveling the disruptive player is carried forward remaining equal in level rather than slowly falling behind to a position where they are expected to carry less and less of the party's load when things are on the line. Under leveling by XP If the disruptive player notices a growing level gap and questions it then the discussion is one about the faults they are in need of improving in and how those faults impact everyone else at the table rather than why comic book guy gm needs to boot them out of frustration. If the disruptive player is unwilling to do anything about that and chooses to find another table then it's easier for everyone else.
When the gm is just following the table then it is what it is for "wow I see myself getting closer " excitement... When the gm is manipulating the table's xp input gains it becomes "wow I wonder how far I am from some nebulous maybe unknowable target". Likewise if the gm needs to create a whole new table it's not longer a thing players can reference in the book and (ime) has a habit of leading to "oops I forgot and was using the one in the book"as the solution chosen by those disruptive players meeting the "find out" part of FaFo.
§ any number of things from unreasonable tardiness/absence unpreparedness etc.. there are practically tropes about disruptive impact of "I showed up" players.
