I've heard that buying the Necromancer pack is going to also increase our stash space by two tabs and our character roster by two. I thought it would be interesting to estimate if that will be enough to store all the extra items we may need.
The bottom line is that 2 stash tabs plus 2 character slots leaves us about a dozen full-size items short on storage space.
Agreed. But making decisions is difficult for many people. And those people are also likely to enjoy chasing achievements. Which makes stash space (and even character slots) an appropriate reward.
And season are, effectively, targetted at keeping exactly that kind of player involved in the game.
I you look at the rest of my posting on the official forums (this site didn't let me post the full calculation), I think I made some reasonable assumptions -- apart from not realising that Necromancers' primary stat is Intelligence, not Vitality.
Let me put it this way: how do you feel if you realise that the build you want to try requires an item you've been consistently salvaging?
Some people will anticipate this possibility by keeping one of everything. Some people will try to avoid it by carefully studying the meta. Some people will get disillusioned and stop playing. Some people will switch back to a more familiar class or build. And some people won't let it affect them, and will simply focus their Kadala/Kanai's Cube gambling on getting that item.
We're all different, and all of these are valid responses.
The producer of a game ultimately needs to understand which types of people are keeping their game alive, and make a choice to address the needs of each type in an appropriate way, or understand the consequences and choose not to.
Personally, I would like to see the opportunity to gain a total of 15 Stash Tabs (this is a bit arbitrary -- there's an argument for four tabs per class, one each for Sets, Weapons, Armour, and Jewellery) and 21 character slots (3 for each class, being one of each class and gender in SC and one of each class in HC). It makes sense to continue to tie the extras to "achievements" and/or (hopefully to a lesser extent) money spent. That way, if you don't feel you need the extra space, personally, you don't have to pursue it.
When you're running fast rifts or bounties, most people want to continue to get the advantage of group play without interrupting the group with long periods of inventory sorting that can be done later.
So it's always good to have a couple of stash tabs clear as very temporary storage.
As you learn which items are most/least useful, and as you increase your proportion of Ancients, the need for this will naturally drop off.