If its anything like the nighthold, as long as you are clearing Tomb on normal you should be able to get one upgrade thingie a week, considering that your legendaries will still be higher ilvl than most Tomb gear I really don't see the problem in it taking multiple weeks to upgrade every single legendary you have.
It ignores such insignificant forces as time, entropy, and death
It's very unlikely you'd get items so good you'd actually think of replacing a legendary within that time. You get to choose the order of your upgraded legendaries, so ones that are better all around you can upgrade first. This isn't reducing how good the legendaries are, they will still work the exact same as they did the day before the patch, you can just make them better by investing a small amount of effort in upgrading them.
They better drop at least 10 a time or people will rage like no other.