Brake fluid change is every 2 years, not 4. But as they won’t touch the pistons or sliding pins it will make no difference to seizing - condition of the fluid doesn’t contribute to that.
Take your point about service costs, always cheaper to DIY (although you have to be careful of fake parts being sold on eBay) but not everyone has the skill/experience, tools, time, space or desire to do that. An independent is a good option as they aren’t charging £150+ per hour nor have 200%+ markup on oil.
But for hybrid owners a Lexus service includes the hybrid check for battery warranty, and now you get a 12 month/10k miles Relax warranty with every service, so for piece of mind it is still a good option for people who’s vehicle is under 10 years old.