Not every movie collectors want are even available in LD. There have been many recent movies, i.e., LOTR trilogy that isn't offered on LD.
That's why I enjoy both formats. Heck, I even own DVHS that offers both video and sound in the best quality offered for home video today.
I'll buy DVDs (sometimes even of titles that I already have) but I do not feel that I must replace EVERYTHING in my collection.
As to laser rot, I am well within my legal rights to transfer a film from LD to another format (hometaping) whether that new format is recordable DVD or whatever.
There are some things I doubt that the studios will release again (especially when there were less than 1,000 copies of some LDs pressed). It was a niche market and was not as prone to controversy.