Speaking of Rashford, he only gives a shit when he wants a new contract lol

Rashford’s a budget player who turns it on for 7 months when a contract is on the line. Xhaka grew as a player when his role was adapted to provide him better cover to make up for his lack of pace and somewhat questionable defensive instincts. Casemiro has just gotten old after being consistent forever.

Younger players who function better in one system or another or others who turn it on at pinch points in their career are not the same as older player going over the hill.

What do you mean about AWB? He went from shit to essential and then has been injured, he's only just back

Rashford and AWB examples are fluctuations in form, an inherent lack of consistency, and/or fit with the manager's tactics and what the squad needs. This is different than an age-related decline in quality when a player hits a point where their experience and knowledge of the game is no longer able to make up for the loss of athleticism.

Outside of significant injuries, it's rare that a good player hits their peak in their early-mid 20s and can never get back to that. A lot will reach their potential and stop developing but they don't go backwards. That's why it's so surprising and attracts attention when it does happen, e.g. Dele.