I see nobody has yet replied and I wonder if, like I, folks are not quite sure what to say on the matter.
I think my first thoughts were to wonder (without having done any checking at all) if your assertions are actually the case, as the broad generalisation you've expressed them as. Are all thematic ETFs higher risk/higher volatility and now trending downwards at speed? Sure, you can think of specific ones where that's true, most famously the ARK ones I guess (although you can't buy them in the UK, 'cos of lack of a KID), but is it true as a broad generalisation? Dunno.
I guess also it depends on exactly what you mean by "thematic" ETFs. Some labelled so seem to be just passive sector ETFs, while others are active.
E.g. I hold the
iShares Global Water ETF (IH2O). Is that a "thematic" ETF? I didn't think of it that way when I bought it, I just considered it a global sectoral ETF. Indeed, it's a passive S&P Global Water Index tracker. (Yes, it's gone down by more than, say, VWRL in the past few months, but on anything from a 1 year view and longer it's ahead.)
Not sure what to add to that ... I suspect that the lack of replies indicate that I'm not alone in thinking that the answer to your post is a big Dunno!
(Now watch lots of strong opinions come in....
)