It is definitely the best way to handle models which won't always be available (otherwise you'd get 'Pay to Win' situations very fast) but I don't think that's the main point people are disappointed about.
It's disappointing that Grombrindal has been built up in AoS so much recently (2 White Dwarf short stories series, one of which was novelized with a bigger story alongside it, a new design, a role in Broken Realms, an appearance in a Gotrek book, etc.) and that there's finally an AoS model for him, and then they make it limited edition.
They could've easily done something like they did with Gotrek. Multi-faction, still with rules, great model to paint and show off too. But this just goes on the pile of limited one-off models they've released recently. Especially in terms of Duardin models we've had four limited ones in the past couple of years. (Jakkob Bugmansson, Torrk Lenssen, Brakki of the Gilded Key and now Grombrindal) Torrk and Brakki should definitely also be sold as Admiral and Runesmiter respectively, they're much newer designs.
That's the same number of limited Duardin models as non-limited ones in the same timespan (3: Auric Flamekeeper, Grimhold Exile, Endrinmaster in Dirigible suit and Drekki Flynt) which is ridiculous and a bit of an insult to Duardin players.