Maybe I'm reading things wrong, but I think they are offering full refunds for 2020 season ticket plans -- it's just that you have to reach out and affirmatively request it. If you don't, then it rolls over to 2021 and you get the 50% bonus.
In terms of the 50% bonus, my feeling is that isn't really the screwjob that some folks feel like it is. For instance, I have a half season plan, but with the bonus am considering upgrading to a full season plan for 2021. Full season seats are a little cheaper per game, so with that "discount" plus the 50% bonus, I'd be paying only a little more to go from 42 to 82 games, and way less per ticket. Finding takers for all those games might be a challenge of course.
The tougher part seems to be for those who already have full season plans, and really can't absorb more tickets. But, for people who are there that much, they might actually be able to use that much for concessions. Say you have a full season plan for 1 seat at $40 per game. Now you have a credit of $20 per game for concessions. (And if you're a family of four, for example, multiply all that times 4). Basically, it comes down to being able to use half your ticket cost on concessions, per person, which until you get into the really expensive seats, seems reasonable.
Assuming there's no 2020 games played at Nats park with fans in seats, then all the 2020 money rolls forward to 2021 anyway (assuming you don't take the refund), so really the only options for a bonus are doing it like they've done it, or else letting you roll it farther into the future, for 2022 season tix. All in all, the 50% bonus seems fair enough to me.
But, I do like getting myself spun up about getting screwed by the Nats, so if others think I'm thinking about it wrong and ought to be offended by the 50% bonus, please help my understand what I'm getting wrong! (seriously)