Boots of Swiftness is a very niche item. It's very good for champions who don't have dashes to catch up to the enemy as well as benefiting those who need the extra movement speed to dodge enemy skillshots. Considering the fact that most Hard CC's these days are Skillshots, I'd rather have the extra MS to dodge them instead of the Tenacity from Mercury Treads.
But at the moment, I agree; it's not in its best state. Within the past few seasons, its stats and total cost were being consistently nerfed without compensation. As for why it's not popular, I believe the reasons for this are that the item lacks gold efficiency compared to the other boots and its passive doesn't feel very impactful. Right now, in terms of the stats it gives, it sits at the bottom of the gold efficiency of all 7 boots upgrades. In order to be completely efficient, the Resistance to Slow Effects passive needs to be worth at least 240 gold, but in terms of how it feels in gameplay; it's pretty underwhelming.
I feel adding HP won't really change the situation to the boots much nor does it fit much thematically as a rush item. For the most part, most people buy the boots to get a 200g MS advantage in lane, which is relatively strong already. By increasing its cost to 1100 gold to match most of the other tier-2s, I still feel that most people would prefer Mercury Treads or Ninja Tabis due to the effective health granted through defenses and to counter the plethera of % damage skills that keep coming out...
Instead, they should increase the base movement speed back to 65, making it a stronger counter to kiting champions and the League of Spooky Ghosts (Twin Shadows) that is coming back in 8.4 or 8.5.
