If I remember correctly Roar is optional unless IMSA requires your specific team/car to attend (in order to get performance data for BoP). Teams which are not summoned are allowed to come and partake for additional testing. At least that's how it's been in the past years. So if you're in the same team/car for a couple years it is legitimate that you chose not to show up for Roar. In some extreme circumstances teams can't even make it because of car delivery issues - not sure how that's handled.
With regard to IMSA, I think DPI is screwed in a few years anyway. Many teams/manufacturers do not want hybrid (since the industry is essentially skipping from hybrid to full electric cars - and full electric racing is nowhere near ready for IMSA style performance). It's also expensive and would lock them into hybrids for maybe 5-years, in which time that tech would be old news.
Numerous manufacturers want DPI to match up with Hypercar....but Hypercar budgets (while theoretically less than LMP1) will still likely be 2-3-5x as much as a current DPI would be, meaning the smaller teams might disappear if they go to a common formula. Also, WEC/ACO/FIA have the huge advantage right now with three major and one minor manufacturer on board for Hypercar in the first few years. IMSA would not be negotiating from a point of strength.
For some platforms, IMSA is currently more expensive than WEC or ELMS (mainly because of the length of schedule and the travel I suppose?). Generally they're in a bad way, with many manufacturers wanting to drop out of motorsport or go all-electric (and again it's simply not ready for prime-time to replace the major motorsport series).
The IMSA racing could be great this year, but they'd have to nail the BoP, something they did extremely poor with last year. We have enough cars for good racing, but not if they are too far apart in BoP (which is tough, particularly in the first two major races).