i don't know what you're talking about football now being focused around the #9 and now more than ever they being responsible for goals, maybe you mean united specifically because that doesn't reflect the reality of most top teams right now. They usually have a bunch of players capable of contributing with their goal scoring endeavors. The burden of goalscoring has primarily been on strikers and it has always been like that, if there was a shift, i'd say it actually was to lessen the pressure on #9 to be the main/only goal outlets. I mean look at teams like barca with a fluid front 3, or teams like atletico and juve with players like griezmann and dybala playing behind main strikers and putting up numbers or even bayern with wingers that cut in for goals. i'm sorry, but your assertion is just not true.
Morata is the perfect modern day striker because he's got great technical ability, can play wide, he's good in the air and capable of seeing a pass and is a good finisher, not just a poacher or whatever. His career goals today do not mean much because generally he has played in much better teams that Lukaku with a higher level of quality and was not the main man, yet he's still done quite well, he's talent is evident, at United he will flourish.
Lukaku's goals come in spurts, he could go 8 matches in a dry run and then score back to back hatricks against fodder opposition to make up the numbers, his all around play doesn't measure up to Morata's and his technical ability is lacking. We should be buying players that can play football because players the overtly rely on their physical prowess like Lukaku usually have a shorter expiration date and are more predictable/easier to nullify.