That's for US searches, and it always seemed that Skype was more popular in Europe, but the graph is very similar if you change it to worldwide searches.

Wow, this is pretty impressive. But where did these people go? Messenger didn't offer video calling until 2015, Whatsapp until 2016.

I simply quit using it. I didn't replace it with anything else.

Maybe a lot were like me, and simply didn't care enough to keep using it.

Apple users stuck to facetime which probably broke up whatever contacts that people had through skype. As an android user I just slowly stopped using skype in favour of discord, except when required for job interviews.