That is wrong - the limit isn't time. The limit is you need to create a good ui so you don't need time. time is the limit for a fun game (see other comnents on how hard this is).

However an app needs to be a good enough ui that it does something for you and often they don't. Paper todo lists still beat all the todo apps people try to make because the ui works for paper. Paper has large limits and so it seems there must be an app out there that is better - but so far everyone has failed to create it.