I'm not sure how much relaying skype supernodes did, but as skype wasnt webrtc-based they would have more control over stream encryption - allowing routing of encrtpyed streams for true e2e encryption.
For small webrtc conferences, treating certain peers as an SFU can certainly work. However, bandwidth requirements would be substantially higher than full-mesh for the supernodes, and equivalent to SFU peers for non supernodes - so supernodes would need to be chosen wisely (and you would still be very limited on number of participants - then you are entering the realms of peering supernodes and intelligent routing of streams). Additionally, this still wouldn't be e2e encrypted. Its just a (current) limitation of webrtc.
For small webrtc conferences, treating certain peers as an SFU can certainly work. However, bandwidth requirements would be substantially higher than full-mesh for the supernodes, and equivalent to SFU peers for non supernodes - so supernodes would need to be chosen wisely (and you would still be very limited on number of participants - then you are entering the realms of peering supernodes and intelligent routing of streams). Additionally, this still wouldn't be e2e encrypted. Its just a (current) limitation of webrtc.