In many cases the newer releases are much much cheaper than buying all those singles.
Of course back when they sold them that way, the releases were usually spaced out over a couple years (or at least one year). Which made it seem less painful.
I have consolidated a few of my older singles down to the newer slim cases, and it makes a ton of difference.
You can always try to sell the singles online, or try to sell them to a used store.
Though some places won't take the singles unless it's a boxed set.
Of course if you are desperate you could sell them to HPB, but don't expect much $.
Or better yet (if you are a generous person) you could donate them to your local library so other people can enjoy them.
That's probably what I will do (if DVDs are still used by then) when I leave this life...of course I hope that is a LONG time from now.
Toho's Godzilla Will Live Forever!