It would go a long way to making Starkiller Base a reasonable weapon if construction began just after the destruction of Alderaan. This way the first Death Star would have been a proof of concept, allowing for the massive allocation of resources to such a project as well as giving the galaxy a feared weapon in the Death Star while their ultimate weapon was being developed over several decades.

After the revelation of the Death Star’s weakness, the official answer would have been to give it an energy shield, and with most of their resources tied up in the Starkiller project, the Empire provided only a quick and dirty proof of concept for the energy shield with the Second Death star. Unfortunately for the Emperor, he got cocky and got himself killed. This had the effect of freezing innovations on the Starkiller project, which kept the planetary shield tested with the Second Death Star but didn’t boast any other defenses of note.

As the Empire lost more and more ground, their ability to complete the project was impaired, so it took 30 more years and a change of management to finally finish Starkiller Base.