I think YT should make some serious changes to its algorithm so it's less easy to "leech" off of popular videos"
That's the main foundation to the site is to be able to freely reply to videos even the popular ones in their own way (as long as they aren't going against the site rules and TOS) even if it's to other users disliking.
"They should also change some of their guidelines and rules"
The main problem with that idea is where would they draw the line? What reply videos would fall into the category of being out right spam and should be deleted and what reply videos wouldn't be label as spam? Why would a video where a user talks about the video they are replying to get labeled as spam and deleted and another users video of them talking about the video they are replying to not get labeled as spam, where the only difference between the videos and users is the first user uploads 20 reply videos a day and the second user only uploads 1 a day but in all and all the two videos are basically identical?
Why stop there. What counts as a "shitty" video? I personally think all the window slide show videos are spam and should be taken down, so should they? Should all reply videos be taken down? Should all videos where it's just the user talking about something like another video or what not also be taken down?
"because at the moment, what these girls are doing is not technically against the current rules"
That's why they are allowed to do it and keep doing it. YouTube isn't going to rewrite the rules to take down all the videos you and other users personally dislike.
Just like how you have the privilege and freedom to make and upload any kind of video you want (as long as it doesn't break YouTube rules and TOS), the "reply girls" also have that equal privilege and freedom as everyone else on YouTube.
So you saying the "reply girls" shouldn't be allowed to make those videos or they should be banned for making those kinds of video yet on the other hand you should still be allowed to do the same time (like replying to a video the same way they do) is called censorship and again NOTHING is making you watch them.