Could dragonless kings exercise a great power as Aegon I and Jaehaerys I? Aegon V could have made it with alliances, Daeron I proved that you can conquer Dorne without any dragon, Viserys II excellent management made possible to finance it after DoD. And, in a world without magic… Stannis wins ala Crecy inspiring to stick with him against overwhelming odds, lose no time at Storm’s End, not enough wildfire to stop him, it does seem like GRRM hints that you can do it without dragons

Well, you have to think about the role of path dependency and institutional inertia. Aegon V, Daeron I, and Viserys II had a monarchy behind them that really couldn’t have been created without dragons – hence the failure to overcome the balance of power between the Seven Kingdoms in the 8,000 years before Aegon’s Conquest. 

The issue is, without the dragons, you’re very reliant on the character and ability of the monarch, and you don’t have much to fall back on if that’s not there. Hence, despite the charisma and military skill of Daeron I or Viserys II’s management, they weren’t able to make enough headway to overcome the erosion that would occur during the reigns of the weak kings who followed them.