The ending was tagged on in post production (voice over narration is always a good clue that soemthing went wrong with the film) after the original cut of the film was deemed not to have enough "useable footage" (to quote Freddie Francis).
Basically, director Sekely made a mess of the film and the salvageable footage came to only about an hour long when it was viewed. Consequently, an entire new sub-plot was written (all of the lighthouse stuff) and this was filmed several months later by Freddie Francis. Howard Keel had already returned to the USA and wasn't available for re-shoots. That's why the lighthouse footage doesn't interact with anything else and you also get the silly idea that triffids attacking a lighthouse are vulnerable to salt water - none of this was in the book or the original script of the film.
You can read all about this in Bernard Gordon's book.
Cynicism is basically a British national trait. Probably due to once being the most powerful nation on Earth, and then losing practically all that power and influence, which has made us Brits realise how illusionary and fleeting power is. It's a lesson America will also doubtlessly learn over the next few decades.
Why can't there be a happy ending?
Because there are very rarely happy endings in real life. (See? Typical limey cynicism, right here.)
When I was a child in the 60's I loved British shows and movies. They had dark endings a lot of the times and that was a contrast to living happily ever after in American film and TV. You never knew what you were gonna get with a Brit film or TV. Also they talked funny. 💂👏