Hi! Yes, all digimon have a lifespan, even megas, when you reach a digimon with no digivolution it will eventually die when the orange D depletes, when that happens it will be reborn as a new digiegg, some of the current digimon stats will pass on to the new digimon. The attacks that the current digimon has installed will be forgoten, but all cards and items you adquire from battle you will keep.
If you have any question you can ask me here or we have a discord where I answer regularly.
Thanks for playing!