This is not a bug, it's a suggestion.
Kumasasa, this site isn't for coming up with workarounds. It's for reporting bugs.
From the wiki: "Contrary to popular belief, lava cannot remain forever with out a source block. It may take several hours, but it will always disappear." It likely will just take a long time, so I would say this bug is "By Design"/"Won't Fix".
That is not a good fix. Lava and water behave completely differently in real life, but most importantly you'd have lava rushing down hillsides instead of moving slowly. Not a great idea at all, in my opinion.