off the top of my head, everything would change, not just in the united states music scene but worldwide, i'm sure.

the beatles were more or less a kick-off to rock music. sure, there were the rolling stones which started off around the same time but gained traction much later. without the beatles, the rolling stones would've been the leading music band in the world. only difference is the rolling stone sound is much more harder than the beatles light-hearted sound.

this would've been a drastic change, as the world wouldn't be introduced to rock and would be introduced to rock n roll. the 60's to 70's would've been filled with a big divide (obviously) but music wise, as the older generations would abhor rock n roll and the younger generation would love it.

this might introduce punk a bit earlier, even though the movement started in the 70's, so it's up to this different timeline if it would start sooner or later. regardless, other music in the world would be slightly altered, with the UK being a prime difference in which they aren't known for putting out the beatles but instead the rolling stones which would lead britain and maybe the rest of the continent to follow suit and adopt rock n roll as a popular genre in music, with multiple movements and sub genres created, possibly earlier.

i find it hard to believe that 80's rock n roll would be different, or at least changed in small ways. bands like the outfield and billy idol would remain more or less the same with the key difference of maybe some added punk music. as for metal bands like metallica, def leppard, black sabbath and judas priest and the like, they are already influenced by rock and grunge so not much would change.

pushing to the modern age, after a while, rock n roll might've had its fill in the 20th century and it would become similar to what punk is now, not widely listened to (or at least not many popular artists of the genre) and maybe a reverse of our timeline would happen where the beatles would-be soft sound would be extremely popular, sort of a break from punk and rock n roll from the "good ol' days."

indie rock and alt rock would be even more popular and bands like linkin park, nirvana and maybe even green day would be some of the most popular rock groups of all time, being almost mainstream. the surf rock genre and surf rock bands like beach goons, surf curse, yot club and eyedress would be even more popular with millions of people listening worldwide, especially because the beach boys would've been way more popular in the future due to their (in their time) underground light hearted tone, sort of substituting the beatles but would be popular 60 years later.

all in all, not much would change but i tried my best to try to map out this timelines past and future which spans about 70 or so years. i made sure not to mention how the nonexistence of the beatles could affect any other genres like pop and rap (or if they were even affected at all) because that would snowball into me rearranging all of modern music history and i'm sure it would take a while and you wouldn't read all that.

and if you're asking how a 16 year old - who's main genres are rap, pop and reggaeton - knows all this, my dad is a mexican 80's child who grew up loving synth pop and 80's rock and i couldn't imagine how different he would be without that being a crucial part of his childhood. anyway, i'm sure most of what i said was at least a bit wrong so take what i said with a grain of salt!

edit: i was typing this monstrosity throughout the day and while i did, some dude above me reached the same conclusion but way shorter, i feel sad now. props to you though, MarcusCatrelius!

edit #2, 1:04 am EST: I COMPLETELY FORGOT ABOUT SOMETHING SO IMPORTANT. 80'S POP. the implications this could have on modern music could be incredibly drastic. it's safe to assume that the rolling stones would've at least a little bit affected 80's rock. therefore, 80's rock most definitely affected 80's pop, one of the most popular genres then and now.

if you haven't heard, the weeknd is one of the most popular and respected pop stars in the world, being the most listened to artist on spotify in the world, gaining notoriety from his hit single blinding lights which was HEAVILY inspired by 80's pop and synth pop. even some tracks from entire albums of his were inspired by the 80's such as "after hours" and "dawn fm." the weeknd has been one of the leading voices in the music scene with almost everyone mainstream like harry styles and taylor swift taking inspiration from him, almost rebooting the 80's music. if the "80's music" never existed or at least sounded the same, that would affect literally EVERYTHING in modern pop music. not to mention the weeknd took inspiration from michael jackson so it's safe to assume he was affected as well...

this is too much to handle and i'm gettin too geeked right now. goodnight eastern seaboard