How much difference in quality is there going to be between a 1080i and a 1080p transfer? Will you notice a difference (when far far in the future they make TVs that can do 1080p). Sure, you'll notice a difference. But will it be a big difference? No, not really. Sure, it'll be there. It'll be great. But if it isn't available, it won't be the same as having to rely on a VHS for your movie today in the era of DVD.
The quality of the originals I don't think are the best anyway. Lowry can obviously fix a lot of that, but we're going to hit an upper limit at some point. Regular DVDs do look great and are eminently watchable on a really big-ass TV (55"+) without too much problem, unless you get into the upper 60-inch range. Then you can really start to see some scanlines again. But with a good progressive player and a quality TV, the DVDs will suffice for a long time. And I'd certainly be happy with them now, knowing they'd never come again, in any format. Will it be perfect? No. Will it be good enough? Sure, you bet.