ilvl is just a number, technically it doesn't matter what that number is. But the larger the ilvl range gets, the harder it will be to balance it with physical levels when those are eventually raised (which would be necessary to balance the introduction of new skills and/or classes).

At the current rate of expansion, I would guess that we'd end up with ilvl100-110 gear before the level cap is raised. To maintain balance, all existing end-game contents up to that point would be synced to lv50, while newer end-game contents would start at lv51-55 (assuming that's the new cap), and all newer progression gear starting from ilvl110 (or whatever) would require lv51+ to wear.

Going any higher in ilvl with lv50 gear and you risk making the older lv50 contents faceroll easy (some already are) and slap people with a huge difficulty jump when going from lv50 content to lv51+ stuff.