Based off my observations:

DD use approximately 1250 to 1600 per second, including executes where it tends to cost more.
Your average recovery is around 1500 with food buff meaning you have a defecit of 1000 to 1700 to cover.

If you use recovery on jewellery at a cost of 550 damage you could boost it by 540 recovery *1.28 armor passive or about 350/second. This would be bad in most cases.
Class resource recovery can be 100 to 400 approximately, depending on the class
Undaunted passive of 4% back per synergy every 10 seconds would net you 140 recovery (based on a 35k bar)
Magicka steal is 170/second
Using a potion is 7500 every 45 seconds, which is worth 167/second approximately.

So, assuming you have a 1300/second cost (that's about where my magblade is I think):
Food Buff + Natural of 750/second
Siphoning Strikes 100/second
Magicka Steal at 170/second
With this I'd be losing 280 magicka per second... meaning with my 36k magicka it would take me approximately 2 minutes to run out if I get nothing else (no synergies, no killing mobs, no potions etc). I don't think many fights are taking 2 minutes where your damage matters too much to heavy attack, you're not willing to use a potion, you have no outside help on the subject providing synergies or other sustain buffs.

I think another concept is to reduce use of your primary resource. For a mage, this means working in a stamina damage skill (since the damage will scale off your higher mag). Perhaps something like caltrops. Or if you need an area heal, vigor.

Well, there are two parts to what you're saying.

First part is regarding sustain (being able to continuously go through your rotation for as long as possible). I think that is in a pretty good place right now. Most players with good builds cannot indefinitely do their max damage, which gives a good ebb and flow to PVP combat in smaller scenarios, whilst also not making those players go to heavy attacks every few seconds to keep their rotations up. In PVE it's a bit different story, what with synergies and healers - especially in large groups - making sustain pretty easy to keep up the whole time. It still requires some attention, not running out of resources, but sustain generally isn't a huge concern for, say, dungeon or trial groups.

The second part you're talking about is recovery (the amount of resources regained passively every two seconds). As someone who has tried to make recovery builds work for the better part of 8 (9?) years, I can say two things about recovery stats. One is that right now, recovery builds are in a fairly decent place in the scheme of things. There are a lot more options than there have ever been to build characters such that heavy attacks are minimally necessary. I don't think that the min/max potential of recovery builds is where it was at one or two points in the past, but generally, recovery is pretty useful now compared to the past. The other thing I can say is that, throughout ESO's lifetime, investing into recovery specifically, at high levels, has generally been considered a waste. It's usually more effective to just mix in a heavy attack, or use an ability that regains resources, over relying on recovery. Especially since if you want to build a character that uses resource recovery stats effectively, it's usually best to go all-in, which lessens your ability to sustain with other methods.

A really nice tip I have regarding both of these, is actually to take your main stat (say, stamina, for a dual wield build) and just max it out, while dumping a decent amount of recovery in your secondary stat (say, magicka). You can use magicka-based abilities every rotation or two, and just use the time in between ability uses to let it passively regen, and you otherwise focus on heavy attacks and such for stamina, while having more than enough resources to keep your rotation up before needing to. This is a great way to make a magicka-based healer, for example, who just needs to roll dodge every now and then, or a tank that just needs a bit of magicka to burst heal.