Iqbal Qasim, Dillip Doshi, Maninder Singh, and maybe a Sri Lankan somewhere too. All superior bowlers to Holland and Border. And while there were indeed a fair few drawn series in the subcontinent, not when WI were involved. Between 1976 and 1986, the period where they flattened all and sundry, West Indies played just a single series each in Pakistan and India, and won them both (1-0 in Pakistan, 3-0 in India, despite being without key personnel on both occasions). Well, actually, they played another in India in 1978/79, but I don't think anyone would suggest that means a thing as it was actually West Indies A playing that series, with the first-team defected to Kerry Packer.
That, to me, is enough to suggest there was no great weakness against spin for West Indian batsmen.
Queen's Park Oval and Bourda have almost never been seam-friendly grounds, and The ARG has rarely offered anything to bowlers of any kind. Generally, spinners had as good a chance as seamers at the aforementioned three. It's only Kensington Oval and Sabina Park that were truly seam-friendly surfaces, and even this hasn't been true in recent years.
No, West Indies haven't had a good Test spinner since Lance Gibbs enjoyed his Indian summer in 1973, 1974 and 1974/75. But that doesn't mean none of their batsmen could play spin.
They didn't "suddenly give-up", but they fairly obviously cared less than they otherwise would have. I may not have watched those games live, but I've seen footage from both of them, and it's fair to say the WI batsmen often looked like they were less than 100% bothered about run-scoring. I believe this would have been different if the games had been live ones earlier in the series.
Obviously they handled quality spin less well than quality seam, but that's not really what's been being suggested - what's being suggested is that all you needed was a half-decent spinner and a turning track and that was automatically game won to you.