I’m betting they’ll cap the current ones at 50 (regardless of it they break them up into their new equivalents or just set everything current to 50) and then introduce a new tier for 50-60. Heirlooms are meant to speed things up on alts; they’re not going to want us heading into Shadowlands day 1 with our mains covered in heirlooms.
The default leveling experience is supposed to be much quicker now, making me wonder how worth upgrading they are. 10 levels right now is very relative to what expansion you are in.
The 110-120 heirlooms right now are statistically weaker than azerite questing gear and only provide an experience advantage when swapped during quest turn ins.
If you can get azerite gear early, or even mid leveling, during the squish, I feel like heirlooms won’t be worth it.
I can see them disabling the traits, but those things, like artifact weapons come at a higher base ilvl than everything else. But you might be right, they could likely be nerfed considerably.