Value decrease will not be a cause for Bitcoin and other cryptocurrencies to die. Value decrease is already the direct consequence or effect of something that might cause the cryptocurrency to die. Legality issues may not really be a problem at all if the project is really determined to continue working. They could easily shift to another country or jurisdiction where they may find legality a non-issue, provided it is not the CEO or any other very important member/s of the team that is/are battling a legal issue o issues. Much better technologies may be a cause of certain coins to die if they are not willing enough to compete and improve.