The lead time parameters are there for many reasons.
If one PPL decides to ignore the requirements then perhaps no big problem - hey what is an extra C172 on the sector loading chart.
However, if one does it then many will do it and hence traffic load planning and many other aspects which rely on the FPL being in the system some time before the departure / join is requested disappear out the window.
There are reasons for the limits specified.
Comply with them until you can convince those in authority that they can be changed can be the only practical response.
The only people who have to file very short notice flight plans are state flights and they are exempt from the requirements.
PPLs heading off on some IFR jaunt have no excuse for not filing their plan in good time.
Regards,
DFC