No that's not it at all. Over the past 3 full seasons, Aguero is basically even with his xG.I think people are misinterpreting the xG statistic, if we got a higher xG than goals scored it doesnt mean that magically our strikers will get more goals, its not luck its about ability.
Thats why Aguero always has more goals that xG expected, he is world class and thats what worldclass players do. They get goals in situations other strikers dont. At the end of the day the xG is a value given from many different players under many different chances but if you have a striker who misses a lot if wont matter if a thousand players would had scored, we will still struggle.
The same applies for our defence.
2016/17 - 20 goals, 22.67 xG (-2.67)
2017/18 - 21 goals, 18.57 xG (+2.43)
2018/19 - 21 goals, 19.95 xg (+1.05)
In fact I'm pretty sure that was the basis of a lot of the initial studies. The best strikers are very frequently not actually anything special in their finishing relative to the chances they get. The thing that they excel at more than any other player is the ability to get in those quality chances, aka get a high xG. Scoring lots of goals 1 year with a low xG doesn't mean that player is world class, it means it's a statistical outlier for pretty much everyone and odds are that it will revert very close to the norm the following season. High xG players tend to be the highest actual goalscorers, and the ones that can actually sustain that year after year.
Also feel like you're referring to our own players like Rashford missing chances regularly, being poor finishers, etc. Rashford since he broke through:
2019/20 - 2 goals, 3.30 xG (-1.30)
2018/19 - 10 goals, 11.61 xG (-1.61)
2017/18 - 7 goals, 5.53 xG (+1.47)
2016/17 - 5 goals, 6.27 xG (-1.27)
2015/16 - 5 goals, 3.03 xG (+1.97)
So he's less than a goal off what his xG says it should be at, over ~3.5 seasons. Standard finishing. Just like Aguero's over the past 3 years. There is always variability, but in general with a large enough sample size, players tend to average out to their xG.