If BA are introducing 3 daily flights to BCN and ALC from Gatwick then why haven't they cut the services from Gatwick? Why would Stansted suffer a complete loss of those routes because of that? EasyJet are competitive enough to maintain capacity to both ALC and BCN from all their London airports despite BA's move.
You may say that they wouldn't cut Gatwick because they'd reduce their share of passengers on the route, but axing Stansted flights gives it soley to Ryanair, who is a much bigger rival than BA to EasyJet. If BA were to drop/reduce those routes in the future, then EasyJet would have a hard time regaining those passengers it's lost to Ryanair.
I very much doubt they'd give in to Ryanair at Stansted because of 3 new daily flights with BA from Gatwick. I'd say the two events are totally unrelated. Plus it doesn't explain why further frequency cuts have been made on other routes including Glasgow, Munich and Naples.