You are correct it's totally subjective. I was always told by assessors to just let the player stay on. However, much like you, I've noticed a few lately being told to leave the pitch.
I read that the guidance is supposed to be that if treated by the medical staff for a maximum of 30 seconds then the player can remain on the pitch, any more and they have to go to the sidelines. Presumably this is so that more involved injuries are treated whilst the game can continue.
Doku was treated for less than 30 seconds, but what compounds the decision is that he then let Doku back on the pitch just in time to steal the ball from the unexpecting Everton player and set up a chance on goal. Can we blame Everton for passing the ball that way when Doku was being waved back on, or the referee for not anticipating the flow of the game? Either way, problem is averted if Doku stays on the pitch which, by my understanding, would have been perfectly permissable under the rules and in fact probably wastes more time by forcing him off whilst everyone waits.
I often wonder, do referees at the higher levels still get assessed on their match performance? I'm sure it's an extremely high pressure role and you can never be totally right, but I can't help but think that clearer communication on decisions would help us understand and not be so critical or feel hard done by.
For example, a mic'd up ref in this game telling us Doku has to leave the field for further treatment because xyz, that's fine. As it is, both sides feel aggrieved because nobody understands which part of the rule book the referee is going by.
For me Oliver had a shocker, and I'm not taking about the big decisions necessarily. He blocked passing lanes so frequently for us that it became something we had to play around and even forces us backwards on a couple of occasions. The two late challenges by Beto, I can only assume he classed them as persistent fouling? But they're both very similar, both 'naughty' challenges on the ankle/achilles that are definitely bookable, regardless of whether view them as clumsy or intentional. I think Everton saw his hesitation to book players initially as encouragement to leave one on us, and that is where player safety takes a hit.
And the corner that wasn't a corner... So frustrating. Seems to be more and more of these which are read wrong for throw ins and corners, I'm not sure why that would be but given the huge focus on scoring from these and the rugby scrum messes in the box that have been cultivated into the game, I think a sharper focus on getting calls right here is required. We're not far off having the title decided by an incorrectly given corner, or a game being won 3-0 all from set pieces that weren't called correctly. The whole issue of wrestling in the box hopefully gets addressed during the summer with the annual rule changes, it's made our game a less entertaining spectacle and actually discourages attacking or inventive play in favour of power and walking the thin line between technical foul and permissable pushes.