No-expiration of miles with Delta is a major benefir from my perspective.
I also noticed the 1-way business flights between US and EU are no more 60.000 like they used to be (last used this way in 2016). :-)
Personally, I'd keep Delta and use the miles for intra-US flights.
And yes, there are many more chances to earn miles besides flying in the U.S. than in Europe, seems to me.