Single class builds ARE viable stat-wise, you just need to realize that there are other items in the game besides blues/purples. This is especially important for summoners due to the sheer power for +pet bonus yellow/green affixes. It was already possible to reach 10k HP on a single class character EVEN BEFORE the great +HP giveaway post-B31. In ultimate, especially, a single affix can grant you as much as +80 points to any particular stat.
It’s actually incredibly ironic that you’re arguing that single class Occultist summoner needs more stats. The one thing that makes single class masteries unique is that they’re sitting on a 32 skill point advantage over a character that has to max two separate masteries, so as an Occultist summoner with lots of extra skill points you can divert your choice of item bonuses purely towards survivability bonuses like physique, health and resistances. Subjugator’s (the affix that grants the highest +%pet damage) only spawns on rings, amulets, belts and medals, so other than those you’re free to wear anything you want.
As far as what other single class builds really need, increased availability of end game gear with +stats/resists is enough like faction torso/pants/gloves/feet gear with +100/100 to two different stats and bonus resists/HP/Energy Regen or the equivalent legendaries - note there are already actually a few empowered epic versions of items that mostly focus on stats and resistances, like the Herald’s Regalia.
What makes planning a single class build tricky is that the build needs to have all the skills it needs from one mastery, as one might be able to deduce from our previous discussions above. I would argue that a DEE Occultist, for example, badly needs a second mastery because it relies too much on one particular skill that simply doesn’t cover everything you need to beat the game (besides, you’ll eventually run out of acid/poison and buff/debuff support skills to level). In comparison, an Occultist summoner basically has all the skills it needs (and more) - why would you bother borrowing skills from another mastery?