This is speculation on your part. Do you know what budget is allocated to D3? If so, prove it. You may be correct but you keep speaking as if you have some insider knowledge on D3 budget.
That is what Nevalistis stated was used for Blizzard’s internal test on the increased stash tabs (it was not a test that some player did). No player ever reported an issue with stash tabs in their dedicated PTR thread that Blizzard created. I remember these things because Nev was responding to what I and others posted.
Also please recall the subsequent PTR where there was a bug and it had 14 stash tabs after the earlier PTR with the whole stash tab fiasco.
https://us.forums.blizzard.com/en/d3/t/14th-stash-tab-on-the-ptr/988/22
She clearly states that Blizzard has “no plans to further increase stash space at this time”, leaving open the possibility that at a future date that another increase in stash space could occur.
I have been looking for posts from Blizzard at the time of the PTR 2.6.5 stash tab fiasco.
Patch 2.6.5 Stash Space Clarifications - News - Icy Veins (icy-veins.com)
The old thread link for the dscussion with Nev (I think) was
https://us.battle.net/forums/en/d3/topic/20771407014#
I tried to use wayback to get its contents and encountering issues.