There are alot of things wrong with hp as a primary defense. For starters, there will be even less incentive to bring in mixed damage, as hp counters all. Also its a giant middle finger to all champs with poor ratios as they will pay more than the defender to keep the damage relevant. Also a giant finger to all burst casters (if any is still played) because APs have no real counter HP stacking and they lack sustain damage. ADCs will be even more end game focused as crits are the only way to counter hp.
So basically its the same as S2 except AP mid getting replaced by AD mid, top even less variety(BC BT and Warmong or gtfo), and carry do nothing except farm till 6 items then penta the enemy team.
Reality is defined by your imagination, the catch is to convince yourself what is and what is not.