Once FP+ is in full use, sneaking in will do more damage than it's worth. Every RFID you pass will know you never entered the park and could alert security. No rides will let you on. You can't get or use FP. You can't order food at some QS. You can't use ADR.
All you could do is watch the parades from non-preferred viewing locations. Not much incentive to sneak in.
This could be what they are going for. Unless you buy a ticket all you could do is walk around. the plus for Disney is drawing in people to buy food and trinkets. The unknown is will they be able to generate enough revenue to pay the live entertainment IE Off Kilter etc.