I completely agree with you on World Class, but I'm in complete disagreement with you on the 'matter of opinion' when it comes to skill.
There's a whole field of Mathematics in Statistics that allows economists, scientists and every discipline in life to make inferences about any domain based on data.
That's exactly where passes per game comes through, through-balls, dribbles in the form of descriptive statistics, and there's a whole field that's recently emerged in football in the form of xG in terms of expected from predictive modelling.
Teams can calculate the expected cost per assist of a player, the expected pound per km run over a season and so on.
Although for fans we see players for artistry, flare we label things like footballing intelligence, that a player is a CDM and not a CAM and so on, statistics can give you an insight into how a player will perform in various positions too.
Obviously there's some variability, and scouts will look at data and also have 'hunches' and some bias when it comes to players too, but I don't think it's opinion based or subjective either.
When you see a player like Lewandowski have a higher xG than other players, when you can predict him to get 45 goals next season and 12 assists, it's not even an opinion anymore, Lewandowski is World Class by definition. Anyone that exceeds him is also World Class.
Thiago is world class, comparisons against Uniteds current line-up also shows he's a step up in certain roles based on his passing, scouts, it's not even debatable.
https://www.footballcritic.com/rankings/the-best-bundesliga-players-in-2019-20-statistically/854
Thiago Alcantara
Also Sancho
Football is Football but it's also distilled to a science now, losing the ball too much in the midfield, not making enough tackles and not making enough chances from the midfield? try and find a player which higher interceptions and pass completion and key-passes.
Those who are saying Thiago isn't World Class clearly have some form of bias, the numbers don't lie.
At 30M he's an absolute bargain, he's the best midfielder in the Bundesliga.