Bringing in a pertinent comparison to a specific point of argument is not equivalent to straw-manning, The point being made is that it is incorrect to assume a player's (or any athlete's) physical aptitude based on their age. Whilst there is a strong correlation between age and physical aptitude, there are plenty of sportspeople blessed with great genetics, which combined with a dedicated, professional approach, can see them possess the physical aptitude at 35 than many players at their peak never had.
In other words, criticising a player's age is a fallacy. By all means criticise their fitness or physical decline, but not their age. It makes no difference out on the pitch.
Additionally, whilst speed (or pace) is useful for a footballer, it's probably the most overrated attribute there is. Decision making and anticipation, that is "deciding what to do" and "understanding what is happening and going to happen" is infinitely more important. Raw pace wouldn't have helped in many recent matches.
Can't agree with your assessment, but now we're mainly talking about purely subjective things. The link up play in many recent matches between Mata, Pogba and Zlatan has been very good in my opinion.
If we focus in on one thing we can quantify however, bad touches, then stats are not really baring this out. Forwards are always most guilty of this because of the type of positions they receive the ball and the types of balls that are often played to them. League leader in bad touches per game is J. King on 3.3, Costa is 4th with 3.2, Sanchez is 8th with 2.7, Aguero is 12th with 2.4, Zlatan is 18th with 2.4. Everyone in the top 35 is an offensive midfielder or forward.
The biggest factor in his lack of goals is likely to be just variation or chance, in my opinion. I don't think enough credence is ever given to this in football.