passive sustain is always up, and also double dips off its own increased healing effect. In combat sustain while situationally higher numbers is, in every case just mentioned overall significantly less than they regen from their passive hp regen.
5 hp a second over a thirty minute game is 9,000 healing. All of those champions will have less healing done than they passively regened. 5% of that healing, then is worth less than 1/20th of the passive regen lost in the longer term for those with high natural health regen.
A level 13 Darius has a base health regen of 4hp/second, his Q can, at max, heal him for 36% of his missing hp. Let's give him 2k hp, and say he's at 1, and hits three people with his blade. He'd heal for 719 damage. the 5% increased healing would account for less than 36 hp, or the equivalent of 100% base hp regen ticking for. . . only 9 seconds. Oh, and he always gets that regen without regards to hitting anything, or even the presence of enemy champions. Don't forget, that passive regen also occurs during combat.
So, in a best case scenario on the champion with the largest burst healing, the passive hp regen is worth quite a bit more.
Sure, if he had 4k hp, was at 1, and got three people hit it would take almost 18 seconds of passive regen; but wait, by the time he'd have that much, he's be near level 18, where he gets 5.2 hp/sec so that almost 1440 damage times that 5% increased healing gives only 71 health, which would take just over 13.5 seconds to regen, and does not require the absurd 3,999 missing hp.
Also, a level 18 heimer gets 8 extra hp a second from 100% increased hp regen, and Gnar gets almost 7 a second. Some champions with high natural regen get SV too, not just ones who want increased self heals. Why should they get their item nerfed this way without compensation?
TL;DR in every scenario 100% increased regen is worth more than 5% increased healing unless you get blown up instantly; also 5% increase healing even on a 1440 hp heal, which only Darius and maybe Vlad can even get is less than 75hp and will not be noticeable except over the course of a full game, wherein it is surpassed in every single case by 100% increased base hp regen.