there never was any real point in getting full-tank unless you needed it to survive atleast long enough to get one spell-rota off.
Going fulltank means you can be 100% ignored while your cc is on cd, if you can afford it in anyway, building 1 damageitem benefits you so much more.
Also, kills arn't the main income, unless you get a good amount off shutdowns or play Draven
most gold usually comes from farming, and most offtanks are boss at farming once they have a samll lead in lane.
if a game ends befor ~30min, it's not a rare thing to have more gold on a toplaner than on the adc.
With tanks income spikeing in the midgame, but dropping at late as they can't farm as much anymore while carrys gain usually the killgold, having proper balance simply won't occure.
If tank-items are strong enough for the late to work, tanks will have a godlike powerspike at midgame, if tankitems are appropiate for midgame they will scale poorly into late,
S2 had a much better system for tanks, as you couldn't easily get health+defstats or tons off health imidiatly, so midgame-powerspike wasn't as great, but once you got 3+ tankitems, they scaled well into the late, as Warmongs gave massive hp+hp/5 once stacked, FoN granted insane hp/5 with a stacked warmongs and great mr, and FH/Randuins just gave the needed armor+as-slow.
A single of those items wasn't a huge powerspike on its own, even two were not massive, but all 3 together realy gave you tankines and sustain, later is massivly missing currently...
in S3, Warmongs gave to much at once, snowballing out of controll if gained early, and Randuins also gave to much effecktive hp for the gold, so a renekton with a early randuins was prettymuch immortal, which was pretty problematic.
so either get back to S2, have stacking items and much stronger synergie between tank-items, or somehow get tanks goldcurve more similar to carrys, otherwise the powercurves will always deffer greatly.
Some people probably actually want very destinct and clearly sepperated powercurves amongst different casses, but i personally think it sucks to have an xperation timer on your class, or feeling like fighting for survive till ~70% of the game is already over, just to turn godmode on.
Having differences is ok, but i'd like to feel somewhat relevant at every stage of the game with every champ, which simply doesn't happen atm.
something that also could really help would be changeing LW to work like a selfish BC, a stacking debuff that grants you %pen vs a the target.
This would decrease your power vs squishys, especially on assains/casters as the first few hits would not profit much from it, and it would allow you to seriuosly hurt tanks** IF** you actually deside to focuse them for a longer duration.