One may note how nearly everything after the bolded part is a description of how Inspiration (RAW) is anything but "reliable". (Though I suppose it's fair to say that Inspiration that has already been earned becomes the most reliable source of Advantage when a PC needs Advantage.)Inspiration is the main source of Advantage that players can reliably get, and that can be a huge bonus. Players get Inspiration from roleplaying their Bonds, Ideals, Flaws and Personality Traits — and any other time you (the DM or the group) think they should. The PHB is quite light on how often people should be inspired. You can hand it out like a precious resource once every few sessions, or you can have people get and spend it every couple of rounds. I prefer the later, since it doubles-down on the narrative aspects of the game and rewards people for living their characters. Depending how much you like FATE, or other meta-currency games you may feel differently.
FWIW, I would have said the main "source of Advantage that players can reliably get" is the Help action. Which is something very important to keep in mind both (a) regarding abilities/features like find familiar, and (b) regarding larger parties (the more party members there are, the more often a PC may feel they can afford to Help their allies instead of a taking a more direct action).