Foils have limitations, including:
1. Prerequisite of good kite skills
2. Expense of initial investment
3. Strapless = mowing lawn or stuck to surface level tricks
4. Strapped = more fun but injuries likely/inevitable
5. Needs water more depth
6. Needs mostly weed/debris free water
LW TTs have the advantage in all the categories above.
But LW TTs have their own limitations, including.
1. Less upwind capability
2. 1-2knots more wind required
3. Requires larger kites (expensive)
And yes, LW TTs are on the decline in popularity vs hydrofoil. But they won't die out like race boards. Race boards had too similar requirements/restrictions to hydrofoils such as depth, skill, expense, and weed free water - all with less performance. So hydrofoils just do everything better than race boards without any additional drawbacks. You can't say that about LW TTs vs hydrofoils. There are just too many niches that a LW TT can fill that a hydrofoil can't.