the reality is unfortunately that you won't get your expansions back for free now. All the 'old' subscribers got one free year when this new service started January 2017, but that time is long over.
Many 'latercomers' here feel/felt cheated and angry that they lost their permanent access to the cards they paid for, which I totally understand. I personally was OK with Goko and didn't know what to expect from this new service by a new company (Shuffle iT, which I now help out a bit, so I'm the first to admit I'm not unbiased).
It's not like the same people you paid money to did something new and wanted more money. The license from Rio Grande Games to make an online Dominion game was always time limited. It probably got extended for GoKO once or twice (I don't know anything about the contracts and forgot about the time span the game was on GoKo), but then at some point it wasn't.
So basically the poeple who sold you the the expansions "for life", when they knew they couldn't make sure that stays true, are the onea to blame (I guess that somewhere in their Terms of Service this was stated, but like you I didn't expect it to go away 'that soon').
Shuffle iT took over the license and started January 2017. Their business model sounds much worse for the player, you only "rent" the cards for a given time, then have to pay again. But it honestly acknowledges the reality that the license could go away, so it would be unfair to promise "for life". From what I understand, the license was last year extended to until end of 2021.
You can play the cards from the base game for free against a computer AI or other players. If you play against other players, the cards used are taken from what both players rented, so if the other player(s) have rented expansions, cards from them will come up in the generated kingdom.
If you have any more questions, feel free to ask, and I hope you'll give this implementation a chance.