About a year ago I have spent many hours splitting up my automations and I have since added many more. But Home Assistant is now providing a method to simply filter the automations according to area, device or entity. So now I am seriously considering to merge all my automations in one file again. Would that be a very bad decision? Is there a good reason not to do this? And how would I simply merge all automations without copying and pasting them individually? I would love some advice from the experts! 
I can’t see the benefit.
Unless you made them all in the UI (which if they are split up you couldn’t have) then you may be able to filter them but you won’t be able to edit them from the automations page. You will still need to find them in yaml and edit them there.
And if they are all in one big massive file then they will be impossible to easily find.