there were other reasons that the beatles broke up besides yoko ono and here they are:
1.a decreased need to be together:when the beatles stopped touring in 1966,there wasn't as much of a need to be together,thus individual members could pursue solo projects.in fact by january 1969 each beatle had done some type of solo project outside of the band.also by january of 1969,john,george,and ringo at least wanted to be out of the beatles.john wanted to do avant-garde concept art with yoko(no shock there!),george wanted to be the third guitarist in a easy going american rock band,and ringo wanted to be an actor,only paul still wanted to do the beatle thing.in fact during the let it be sessions,george openly talked about wanting to put out a solo album.
2.the death of brian epstein:with brian epstein's death,the beatles were put into the uncomfortable position of having to look after their own business affairs(this leads to big trouble later),in fact one of the reasons the white album took so long was because while doing album sessions,the beatles were preoccupied with apple business.john lennon admitted that he knew that the beatles were in trouble after brian died.there's an anecdote about brian bringing a visitor while the beatles were in the studio and he tried to impress this guy by making a musical suggestion to john,john famously replied"eppy,you look after the percentages,we'll stick to the music."i think george harrison said it best in the anthology when he said"really all we could do was write songs and be beatles,successfully."
3.the rise of paul mccartney:you notice that most of the beatles albums and singles after 1966 were more paul than john,plus mccartney's drive for perfectionism drove both his bandmates and studio personnel crazy(ringo quit for 2 weeks during the white album and also one of the engineers walked out as well)in fact during the let it be sessions paul would often treat the others like his backup band,telling them how to play what to play and when to play(which really pissed off george harrison)
4.yoko ono:john lennon used yoko ono to do that which he could not do:destroy the beatles or at least his image as a beatle.look at some of the songs that john wrote after a hard day's night(like i'm a loser,i don't want to spoil the party,run for your life,girl,baby's in black,no reply,and others),as well as the whole "jesus" furor,these were all done by john in an attempt to destroy his beatle image,but it had gotten so big that he couldn't do it alone,he needed help:enter yoko ono.john felt trapped by his beatle image and used yoko to free himself from it.the other 3 may have been able to tolerate yoko had she kept quiet,but alas she did not.she constantly offered her opinions about the beatles's music and what made things worse was that she spoke with the conviction of the ignorant,as she knew nothing about rock music and it galled the hell out of paul,george,and ringo.
5.business problems part 1 apple:seeing that their nontraditional approach to apple wasn't working the beatles realized that they needed an actual business manager.paul wanted lee eastman and despite the fact that he had good ideas,his candidacy was opposed by the others on the grounds that paul was basically running them musically and by putting someone in charge who was going to become his father-in-law,paul would have way too much control over the beatles.the others wanted allen klein,this in spite of his less than stellar reputation,i'll credit klein for this,he got the beatles a royalty rate that was equal to their status in the music world.john would later admit that the reason he went for klein was to get at paul.
6.business problems part 2:northern songs:in the spring of 1969,lennon and mccartney were in the position of having to prevent an outside takeover of their publishing company.now this was supposed to be a time of togetherness right?wrong!you see to fight the take over,they had to put up their shares of northern as collateral.now in theory they were supposed to have equal shares,but it was eventually discovered that mccartney had more shares than lennon and what was worse was he didn't put them up(klein actually had to put up his shares of mgm to help fight the takeover).that discovery left john feeling sold out and betrayed by paul,thus i think this was one of the factors that caused lennon to deliberately blow the deal
7.the rise of george harrison:by 1969,george was contributing more and better songs to the beatles and while he enjoyed the respect of his peers,it was hard to come by in his own band who treated him like a second class citizen and a kid to boot.this came to a head at twickenham studios:whereas george was coming up with a song at least every other day,john came up with almost nothing and he was resentful of george for this.george,in turn,was pissed at john's seeming lack of involvement with the project,the fact that yoko spoke for him at all the group meetings,and the ffact that john had aired some of their dirty laundry(their business problems)to the media.it was this exchange that ultimately led to george quitting in jan of 69 and not the argument we see with paul in the let it be film.
when i 1st got into the beatles i was willing to go along with the myth that yoko was solely responsible for breaking up the beatles,but then i did my research(as you can see)and i realized that yoko was one of many factors involved in the breakup of the beatles.so in answer to your question:yoko didn't ruin the beatles;she was just one of the many wise and experienced tour guides the beatles met on the road to splitsville.the truth is that the beatles were on the road to breaking up long before yoko ono showed up.