mixed opinion on that, actually
You might see ISDN as a typical telco take on this newfangled data thing. But it does have upsides, if only people understood what they were dealing with.
Over here (and that's very much not BT) a single BRI costs just a bit over a single POTS line and can handle up to two voice channels simultaneously and up to eight numbers assigned to it for a small surcharge (the first four don't cost extra). That's just peachy for, say, a bunch of otherwise unrelated housemates, or a small office, or something. Using it is actually cheaper than two POTS lines nevermind one for each tenant. And it's useful as a fall-back data line. Or as a dial-on-demand setup before the advent of xDSL. Or an easy way to give the fax, the garden shed, or say the home office a different number. Maybe get a separate statement for each number, simplifying billing for the customer. That sort of thing.
But as a telco, it's not the extra capabilities you're selling it for. You don't even sell it as "two lines". It simplifies your own infrastructure as it plugs pretty directly into the switch. And with two data channels you might see more revenue as well as better copper line utilisation. Trust BT to overprice the thing so that it doesn't see much uptake at all.
As "two lines" ISDN to the home never had a solid business case so it was pretty silly to try and have people pay through the nose for the possibilities of services they didn't understand nevermind thought they needed. They just need a phone, one that works.
Other than telco blundering, ISDN isn't quite dead. Besides that BRI, a PRI (the 30 usable channels variant) is what you get if you need more than a few voice channels. That's ISDN too. And, hm, GSM has copied a bunch of its signalling. So at least parts of it see a lot of use.
The technology works pretty well, but the supposed killer app wasn't and the marketing was worse. It also doesn't try and supplant POTS nor claims it needs blank cheques to make that happen. So it's not half as bad as DAB, if you must compare.