It's not instant and it relies on a private company for settlement. Even though customers typically get this for "free", the institutions themselves are paying interac for these (so the account-holders are paying for this one way or another)
From what I can tell, e-transfers will continue using interac's network, so I'm not sure if this will benefit users at all. Depends if banks integrate a new user-facing money transfer option. I suspect it'll take years if not decades for banking institutions to roll this out to users, though they may use this behind the scenes for inter-institutional transfers without Interac.
Someone who's more knowledgeable about Canadian banking and payment processing, feel free to jump in here because I'd like to know more as well.
You're correct on the first point, and it's worth adding that Interac is a closed network - Interac has no obligation to let anyone participate, and it's been a pain point in the FinTech world that you either have to convince them to let you in (e.g. Wealthsimple) or piggyback on a banking partner (e.g. most other FinTechs.) Maintaining that banking relationship is a big burden for fintechs, especially if you rise to a level where a bank views you as direct competition.
RTR's membership requirements are set by PayCan, who have a mandate to foster competition. Any FinTech who can meet requirements can, in theory, hold a settlement account on RTR and participate. Interac has an incentive to keep the club small, so to speak, and prevent new up-and-comers from cannibalizing their market share.
From a competition lens, the hope here is that RTR levels the playing field for both the big banks and fintechs and opens the door to a lot more consumer-facing innovation. That will, of course, depend on the network effect and who gets onboard.