A potentially unpopular opinion here, but I'm beginning to wonder if Rashford is finding motivation hard to come by because he's achieved what he needed from the game i.e. financial stability for him and his family.
It sounds horrendous, but the same school of thought has been directed at a few South American players from very poor backgrounds in the past who made it big and dipped sharply afterwards.
The thinking is that for some of these kids, the hunger to succeed was never about greatness, legacy, or the love of the game. It was about something more fundamental to life: survival and an escape (for them and their families) from a terrible financial state. Talent was basically a lifeline.
When Rashford talks about his charitable initiatives, it's clear poverty was a dominant running theme for his family during his childhood, strong enough that it drove him to accomplish some seriously impressive results for not just his family, but families across the UK. It's possible that it is the same passion that drove him on coming through the ranks and earlier in his professional football career.
As it happens, today he has firmly exorcised those issues (spoken with Obama, has bested the government itself, has unbelievable commercial appeal), and perhaps deep in his psyche, so deep that even he is unaware of it (or unwilling to admit it anyway), his fight is done and he just can't find the motivation to keep pushing himself at the highest level.
This is not me trying to dump on Rashford, it's just a POV that's a possibility