Dunno if it’s worth it or not – if you really want one then it probably is. What you should do, though, is put to bed the ‘will MTB’ing have moved on masssively’ idea. Mtbing develops at a snails pace – there’s a steady stream of bollocks introduced to the market to keep us all entertained, but true innovation happens but rarely – especially if you’re talking about HTs.
A high end HT frame from 1999 would be spot on today – would be just as awesome to ride as ten years back. You can keep up with all the changes in components in any case as things wear out.
ETA: Sry, missed the fact that you’re looking at FS. A FS from 1999 would likely be garbage, true. One from today will likely hold up much better as designs have settled down, but I don’t fancy Ti for FS bikes full stop. Not a great choice for a once in a decade purchase IMHO.