Personally, I don't take away anything from the 90 day run during the summer of 2021 as evidence of anything. That was a pretty last minute decision to bring fireworks back and after they had already made infrastructure changes for Enchantment.
They are clearly putting effort into this show by adding Main Street projections; I don't think we will see the rest of the show in partial form. This show is going to run for longer than Enchantment did. Probably a lot longer.
And honestly, since they are clearly doing this (along with painting, cleaning, new lighting, etc in Tomorrowland) as part of the TRON media event (as
@ToTBellHop has said for a long time now) I wouldn't be surprised to see 2017 pyro run for the summer and then they scale back to 2019 this fall. They will also be filming. Not saying it will happen but wouldn't surprise me in the least; especially if they are concerned about waning demand.