I believe O2 only provide 25GB of fair usage within the Europe Zone on all plans apart from their top of the line plans. With EE it is up 50GB of your allowances under Roam In EU or Roaming Pass ROW (Zone 1).
And in places such as the US, those plans could have some interesting clauses around traffic management and data speeds that you may want to look into.
I am unsure, worth looking into, I did read something to the tune of "Also, there's no usage limit on data, but speeds are capped at 2Mbps (that's fast enough to stream content on your mobile without any buffering)" on their site in relation to some of their Roaming Plans.
I am an EE customer like yourself, just trying to explain it all as objectively as I can to help you make an informed choice on how to proceed.
I personally believe the introduction of Roaming Pass ROW makes EE one of the better networks for the range of countries available in Zone 1 (such as Turkey, China, UAE, Thailand etc, and the previous countries such as the USA, Australia etc) and the generous fair usage policy compared to some other providers, especially if you can get it for the right price or at no additional cost as many have been able to do over the years.
Ultimately the choice is yours. There is a lot of value in your current plan (especially when you have 3 Inclusive Extras which are flexible). I would just say to make sure that any decision you make on what to do going forward is a considered one based on what else is out there for your needs, and not just an emotional one.
