Late to the party, but I had these worries too one day.
During the Novatac era, Novatac bought out machines and parts from the early HDS EDC Basic/Ultimate series, with the understanding (or agreement, don't know) they'd continue the lifetime warranty on these old series...well we know Novatac ended up bleeding money and not honouring the warranty. Warranty that HDS (Henry) said they will honor when they heard Novatac wasn't doing it anymore (and to this day, still honor, within the limit of what they can do without the parts or machines, since these were sold to Novatac).
So yes, the warranty of a product is only good while the company is up and running..as this was already pointed several times so nothing new lol. My point is...and it depend on individuals, but people tend to change and get fed up with their stuff. I have a friend who took a (pricey) 10 year warranty on his car 3 years ago. Guess what, he's now looking at other cars to replace his, even though 3 years ago he was adamant he'd keep the car 10 years. People change, and the consumer society is pushing you a fast turnover.
But is this true for HDS? I've had HDS lights since 2012 (also I first subscribed on this forum in 2006 but my account was lost at some point, meaning I took a while to jump on the HDS bandwagon), and been using many models, including the old Basic/Ultimate. Did I get fed up at some point and wanted a change? Sure, but I always came back to HDS...making me spiralling in the worries of my favourite brand disappearing one day. I am firmly convinced HDS models from 15 years ago (I was going to say 10, but looks like I aged with the lights too!) still have a point, and I have one right now as emergency light in my hallway, my better half is using a Novatac as night light, I have various Novatac/HDS around...and still using them.
If HDS were to disappear (and I really hope this will happen only after my own disappearance only, cause, you know, I'm selfish too!), I will still have my lights. A few failed during these years, got repaired or replaced, but most didn't. I think of them like tools : along the years I bought mechanic tools, and I'm not easy with them. Many are with me, some broke, got replaced, or I bought replacements from different brands, but the core of what I bought like 20 years ago as a starting kit is still with me. Most carry a lifetime warranty too, but if tomorrow Facom or Stahlwille, Hazet, etc go bankrupt, the warranty is gone. Same is valid for Surefire, in the light of these last months, who knows what can happen, and we've seen it can go fast.
So in the end I hope you'll pardon my rant, but there is no good answer to the question. Buy HDS with confidence, like you would buy any tool carrying a lifetime warranty with confidence. It may fail, but the fact that it comes with a lifetime warranty tells you the manufacturer had faith in the product, or they wouldn't be in business anymore (and yes, to keep with the tool stuff, some fishy brands offer lifetime warranty but try everything to deny it, not the case of HDS)! Like many electronic items, I too noticed there is either an early fail, or a very very late fail.
What will I do when my Novatac will fail (supposing I live long enough to see that)? I'll be sad, as it once carried a lifetime warranty, now gone. But that was many years ago, and I since learned/got used to consider manufacturer warranties with this grain of salt that time passing gives you. But had Novatac still been in business today, wouldn't have I wanted to try something new instead of going through the "hassle" of getting that "old light" repaired? Like a new led, more modern, higher CRI, more lumens (the night is obviously getting darker), new UI, etc ? There is this fine line in a product's life that may make you want to change...and that depend on the product itself, as on the individual buying it.
So I got older and kind of stopped worrying about that. After all, when you reach the point of having 10+ HDS, every one of them that you buy helps easing your worries a bit