By nerfing base hp and hp regen as well as their scaling they are:
making him more vulnerable to his opponents' burst damage at early and later levels
making him more vulnerable to being slowly chunked out when his opponents do manage to get a good trade onto him
Basically, they are making his laning phase weaker by rewarding opponents who get off proper trades vs him by making their damage on him more meaningful. So uh... they are doing exactly what you suggest, just not in the exact method you suggest. And here's a consideration for why they SHOULDN'T do it the way you suggest:
Think about nidalee in mid and late game teamfights. How often do you see her hit a spear on a bruiser or tank or even a riven and decide to go all-in off something like that and actually get positive results? Basically never right? Yeah. Because she's a squishy vulnerable ranged champ that relies on her range and strong poke to build an advantage over you so she can EVENTUALLY go in off a good ranged combo and finish you off. Well that's kinda the same story with jayce in ranged vs hammer stance. If his ranged stance is made significantly weaker UNLESS followed up by an immediate melee combo, then so many champs would just wreck him for that during the early phase of the game when he's supposed to be stronger than them (if played properly) and also they'll outscale him.
They already incentivize him to do melee through a combo of it offering good finishing damage on opponents, disengage vs certain melee's like renekton, and to restore his insanely low mana pool compared to the mana pool of other such mana-hungry champs. Seriously, could you imagine if ezreal had the mana challenges jayce does? And before you go thinking I'm some jayce main or enthusiast... you're more than welcome to research me - I basically NEVER play jayce. In fact, I'm generally the guy playing AGAINST him up in top lane. So speaking from THAT type of first hand experience, I can tell you that putting some sort of further incentive for jayce to use melee over ranged form for laning phase power would neither be of good service to his opponents nor jayce himself. Either the melee bonuses he'd get would be so great that you couldn't touch the minions away from your tower because jayce would out-duel you in both melee and ranged to such a massive degree or... the ranged into melee combo would work much like nid and while offering a good bonus would not work due to how squishy the guy is vs a much more durable bruiser or tank and he'd just get wrecked for gap closing and swapping to melee without properly poking someone down enough first.
The changes to hp and hp regen mean getting a good trade off vs him now increases your ability to win out in the game of trading your hp bar away for his mana pool. They make it require fewer trades vs him to get him low enough in hp that you can all-in him. And they leave laning phase jayce in that position of power that he needs to be viable beyond laning phase IF he's played properly. I don't want him more feast or famine from laning phase on and becoming another problematic champ that either needs regular fine-tuning ever patch or so or else to be left ignored and be largely unplayable or very weak for large portions of a season. Which is exactly where he would be by making him more melee focused or even ranged-into-melee combo focused for laning phase power. And while you might not know it yet, you yourself agree with me too.
