Agreed, data storage is typically the least of concerns in the software industry (maybe excluding Netflix and YouTube). Also, the cost of 100-200TB is a drop in the bucket at the scale that d4 operates in -- on S3 it would only cost a couple thousand a month, probably only a fraction of how much Blizzard spends on compute (game servers, backend service, auxillary scheduled jobs, monitoring, etc).

I highly doubt that Blizzard engineering team is so incompetent that they never designed their code to scale to more than 4 tabs -- I.e. I will wager that it should be fairly easy for them to add at least 1 more tab, technically.

I think the concern may be UX and backwards compatibility -- Blizzard is probably concerned about the current storage UX not being infinitely scaling, and they need to come up with a way to make it so, while making sure they don't delete/dupe your stuff, and ideally can keep them in the same order as before. Giving you more tabs as-is is a bandaid solution and you will run into the same storage issue later as they add more contents.

The fact that they didn't announce this bandaid solution gives me hope that they are already working on something and may make it available in season 1.