Currently, I only have experience with two other games in terms of Expansion content, WoW and Guild Wars, and both of them have been fairly strict on opening old expansion content. For WoW, the current expansions have been bundled into the base game roughly a month prior to the release of a new expansion since as far back as Warlords of Dranor. If you wanted to catch up before the new expansion before then, you needed to buy the current one as well (while prior to WoD, you had to spend ~$60 + the new expansion to catch up). Guild Wars on the other hand seems to prefer doing sales of their old expansions during the release of new ones, allowing you to bundle the older expansions into your new purchase for a slight discount. Opting out of that forces you to pay the full price for each expansion separately.

As far as I can tell, Shadowbringers not unlocking old content until release seems more or less par for the course, only being slightly worse than WoW's release method and definitely better than GW2's method. At the very least, you won't have to buy Heavensward as well as Stormblood if you want to pick up right away. It sucks, but you'd be hard pressed to find anything more generous than this in the market today.

Finally, a quick search revealed Stormblood to be around $25-$30 on amazon depending on what platform you choose to play on, so at least there's that.