what if the end user and source user have the same owner, that is using another of is pc in a different location or many pc in different part of the world to keep his activity secret? i would you know where he is really located?
if you are a agency looking at the end IP.. trying to find the source is hard..
but if you are the ISP knowing every move every source user is doing. it gets easy.
why do you think the "snoopers charter" exists. that governments are requesting ISP's to keep logs of all users AT SOURCE.
we are not talking about trying to locate millions of users using a certain port. we are talking about UK ISP's ability to notice just 230 nodes using a certain port and sending certain size packets of data via them ports. we are talking about US ISP's ability to notice just 1500 nodes using a certain port and sending certain size packets of data via them ports.
you would be surprised how easy that is to do.
many people thing "the whole internet needs to crash to stop bitcoin". or its impossible due to proxies can change in a second.
but if 5000 nodes are physically disconnected from the internet. you will notice it. and the billions of other people who dont know what bitcoin is wont be interupted.