Since Vaal Pact/instant leech are intended to be gone in 3.1, traditional defenses like armour, block, evasion, dodge or MoM will become more important. Casters are poorly positioned on the tree to get four out of those five. Furthermore, the intended new location for VP is extremely bad for casters, so that it might not even be feasible to pick up VP for the doubled max leech rate. In other words, casters are looking like they will be stuck with a leech cap of 20% of maximum life (or ES) per second and little options to get meaningful amounts of evasion, dodge or block.
This is an issue since the two defensive mechanics which are clearly intended for casters, namely mana stacking with MoM and ES recharge, are severly lacking. ES recharge is interrupted whenever we take a hit and therefore has a massive anti-synergy with clearspeed. Getting hit while engaging mobs is impossible to prevent, particularly on melee-range casters like BV, DP, and so on. Constantly leaving battle to recharge ES is just not a feasible/competitive playstyle compared to attack builds.
Likewise, MoM has massive anti-synergy with using auras, a drawback that attack-based builds dont have to deal with. And the mana costs of casting our spells eat into the 20% of maximum mana per second leech cap.
Overall, casters are already at a big disadvantage in the current 3.0 meta for a variety of reasons. Removing instant leech will only make things worse because the tools to compensate this loss are much worse on the caster side than for attack builds. Thus, we are at risk of a completely lopsided meta in 3.1, where casters are all but extinct. The current "caster defenses", which we will have to rely on more when VP is gone, just arent cutting it. Please GGG, deal with this issue!