Stasis wards aren't impossible to counter if you have decent micro
For example, you can just send a few marines or any ranged unit in fact, and shift queue attacks on each stasis ward which will kill off most of the ones not being protected.
And like you said, diamondbacks can be used too to clear out a large area of stasis wards. Though you're correct in saying it will only be a short time before they're repopulated, it's about providing that window of attack using the diamondbacks. You can clear out a large amount of them to set up for your main attack.
Nonetheless, I think stasis wards can be a fine temporary choice to defend against rushes as an alternative to anti-unit tower but does get slightly ridiculous when you're able to spam them throughout the whole map, as all they do is prolong the game as opposed to providing a tactical advantage
So my suggestion is to set a limit on them per player, thinking maybe 3 or 5? This way they will remain as a decent solution against rushes.
As far as melee units are concerned, as a player who's warranted so many nerfs to the ling/rine build, I don't see a problem with them right now
![lots of laughs lol](./images/smilies/icon_lol.gif)
. People have called them OP when I use them in scrims
![dunno [dunno]](./images/smilies/icon_dontknow.gif)
anyway if you're trying to use melee mercs without any marine support, then it obviously won't work out as well as ranged unit composition; think about it this way: in an engagement of melee mercs vs ranged, the melee mercs will only have a few attacking at an instant, the ones who are covering the surface area of the ranged army, but the majority of the ranged units will be attacking at once. This changes when you use marines behind them and change the role of the melee mercs to tanks. If you're implying that temple zealots aren't tanky enough you should watch some endgames in high level scrims, as they're one of the most used endgame units and it's tough to kill them
If you're so inclined to use melee mercs without marines, try to go for rushes to their base, snipe merc compounds, force saviors, surround and flank, etc. there's a variety of ways to utilize them; don't go for frontal attacks as they'll probably just mow you down, unless you have a strong lead or you counter them