I'm going against the grain and saying I actually doubt that SHINee have a song prepared in case of disbandment or in case they didn't get support as 4 after what happened. The group has always been a brotherhood that continues on despite what happens that may leave them with less members, and as they are the type of group that has gotten close like family (one of the few that genuinely works as a family and treats one another as family as well as their family treating each other as extended family) I don't think they would ever "prepare for the worst" in disbandment.
I think they have likely thought about it happening due to military prior or other circumstances, or after what happened, but as far as actually making one or preparing one? Nah. They would be the type of group to sort of....fade away if they disbanded, but, they'd still continue to be close personally and probably attempt to reform etc in the future down the road. They are lucky enough to have made enough money and continue to perform and release things together even if there is minimal support.
As far as others, I'm not sure - I don't think many really think about it outside of "well this is a trend" or "we are it for now but at some point we probably will disband and move on" etc - most companies don't plan for disbandment (unless it's a purposely scheduled group like Wanna One etc) and those that disband groups on their own (without members just leaving due to contracts expiring etc) due to finances or what have you, move quickly to the next. The group members themselves are the ones that typically want to create tours, or albums, or songs, or fanmeets etc to celebrate having worked together/goodbyes etc it seems.
I think the majority of groups and their companies view their work as a part time job that they one day will quit and move to another part time job - not as many have the views of having a long, fulfilling full time job that they will do until they retire.
That was obnoxiously long, but, I hope it made sense!