So you're saying that the existing, observed time dilation effects of spacecraft aren't real?
Photons aren't the only thing slowed down by time dilation. A photon traveling in a straight line is just a simple way to visualize it. Look at chemical reactions: chemical reactions are basically just an exchange of electrons, and the electrons themselves must move slower in their orbits if the nucleus they're orbiting is moving at relativistic speed.
Its all implied by the fact that the speed of light is an absolute speed limit in the Universe, and this light limit requires time dilation. You simply can't have one without the other.
Think about this: if a simple body is moving at exactly 1*c*, all of the motion inside of that body is tied up in its linear motion. How could electrons orbit their nucleii if they're too busy moving at the speed of light along the object's path of motion? The body would have no temperature, no electrical conductivity, no internal chemical or electrical reactions of any kind.
Now reduce that to 99.5% of the speed of light. Rather than all of an object's energy being taken up by moving forward, a mere 10th of that object's energy can now be used for things like orbiting electrons and molecular vibrations.
That's all time dilation is: it's just the internal reactions of an object or system being slowed down because the light speed limit itself is limiting those reactions.