I suspect you're underestimating the ability of criminals to obtain communication tools with sufficient plausible deniability to prevent detection by the local police after the first few prosecutions for that.
All they really need to hide from the local cops is an app that appears to be something else. A quick google search for "disguised encrypted chat app" found one called CoverMe that can disguise itself as a photo album. There are probably more sophisticated options available now, and there will be an explosion of them if the EU bans encrypted chat apps.
With a marginally more sophisticated user, they can get far more hidden. The Android anti-theft app Cerberus, before the company behind it imploded spectacularly, could be installed as a system app on any rooted device, then hide itself until a user-specified code was entered on the phone dialer. If there isn't already an encrypted chat app with that feature, there surely will be after an EU ban. The barrier to entry is not high.
I'll grant it would likely result in a small number of gang members spending a greater percentage of their lives in jail, but that's not a lot of benefit for an extremely high cost.
