Is the speed of light a limit — or a boundary?

I’m proposing a conceptual (not mathematical) hypothesis and would appreciate critical feedback, especially from physicists.

The usual interpretation of the speed of light (c) is that it is an upper limit on signal propagation within spacetime. I want to test an alternative reading:

Hypothesis: c marks a boundary between two physical regimes, not merely a limiting value.

Below c, motion is a property of material objects describable within spacetime.

At c, the notion of a material object ceases to be well-defined (no rest frame, no proper time), and only relations/processes remain meaningful.

This does not propose new physics, superluminal signals, or violations of relativity. It’s a reinterpretation of what changes in kind at light speed.

Motivation: photons have no rest frame, proper time collapses on null trajectories, massless and massive entities form disjoint kinematic classes, and causal (light-cone) structure seems more fundamental than geometry in several modern approaches.

I’m explicitly interested in falsification:

What existing results, or what conceivable experiment or theory, would decisively rule this boundary interpretation out?

Thanks in advance for serious criticism — agreement is not the goal.