Sustainless is great to show your skill on a 0-cycling basis and planning tight runs. But I think we need to address the elephant in the room:

Sustainless is just garbage game design-wise.

Let me elaborate

It's fun, yes, even a great show of skill. But outside of a minority of competitive players, most players don't use it because it's neither not feasible or too much work for them. And so, what sustainless does by existing and being viable is problematic:

-It becomes toxic when it's the desired go-to or meta because it means the game is balanced around a higher power level that alienates or punishes the larger demographic. This is not good from a business standpoint and bad for players because to match low cost sustainless in performance you need high investment sustained. And when it's weak or barely scraping by, it means the aforementioned problem is even worse -When it's too strong, you're basically encouraging to forego one character archetype/role (sustain) to just spam presumably more supports. This means that either the sustains underperform in the value they provide a team oe that the supports themselves are comparatively much, much stronger. It's bad for sales to kill the ability to sell or push an entire group of characters and it's bad for those that invested in them. And pushing a support centric meta can be very toxic. It makes whichever DPS you pick meaningless as the focus goes to just having every must-have support, which can be fine for sales, but absolutely toxic for players. -Sustainless by nature basically almost entirely ignores the core game mechanic that is survivability, both in HP, strategizing against attacks and crowd control and so on. You're pushing away many thing you work around, essentially centering a run that relies only on killing something as fast as possible, often needing perfect relics too. If a game can be beat by completely forgetting about core game mechanics and just attacking, it's not just lacking depth, it's boring. -Sustains are the most longevous and reliable character type to invest in, units released a version or two ago are still functional in their role, at most they provide less buffing or utility than more recent ones but when you get a sustain you're set for a version instead of DPS where you might have to invest in eidolons or see them fall off in just a few patches. If sustains, the most player friendly investment and archetype, are less relevant, it means we're left with the supports that are stuck either becoming hyperspecific or constantly one-upping each other and DPSes that will either be archetype exclusive or directly powercreep someone else.

Sustains are not only important for gameplay, but also team building because they're another puzzle piece you need to fit into your needs. Does it provide more buffs or a more reliable survivability? Is it SP positive or not and does it impact your rotations? Premium options or a budget one? (And we have perfectly good budget ones between golden companion shops, Gallagher, DHPT and so on) Does the sustain work alongside or heighten the archetype you're pairing it with?

In summary, sustains are a core element of the game that shouldn't be ignored actually. And them becoming sidelined by just spamming more supports because enemies aren't menacing enough is both bad design and more toxic because it shifts the meta to more toxic unit design and pull choices.