The thing that really matters when breaking thing apart into ever smaller things is not really if we are accessing ever deeper level of reality.
What matters is: can we use this newly discovered sub-structure to do something we couldn't before.
The answer to this was a clear and resounding "yes" when we reached the level of molecules (chemistry, which allows us to do a great many useful things), still "yes" at the level of the atom (atomic energy, transistors, etc...).
It is however unclear that QCD, quarks and inner proton structure reality level have yet produced anything usable to implement our will upon the world.
It may yet happen, but to answer your questions: once the depth we dig at stops producing anything usable by an engineer (string theory, quarks both currently fall into that bucket I think), not entirely sure the digging is philosophically valuable in any way.
What matters is: can we use this newly discovered sub-structure to do something we couldn't before.
The answer to this was a clear and resounding "yes" when we reached the level of molecules (chemistry, which allows us to do a great many useful things), still "yes" at the level of the atom (atomic energy, transistors, etc...).
It is however unclear that QCD, quarks and inner proton structure reality level have yet produced anything usable to implement our will upon the world.
It may yet happen, but to answer your questions: once the depth we dig at stops producing anything usable by an engineer (string theory, quarks both currently fall into that bucket I think), not entirely sure the digging is philosophically valuable in any way.