> warranties took this into account and it was a normal procedure to add RAM, disk drives and video cards etc. without voiding the warranty

Again, very limited warranties that only covered manufacturing defects. Not the warranties integrated products have today. In most cases, a manufacturing-defect warranty is not voided by rooting your device. (It may become more difficult to prove it’s a manufacturing defect, however. The law varies state to state.)

What fundamentally changed is warranties expanded as products became more integrated and the market expanded beyond power users. You cannot provide accidental-damage insurance for a user adjusting their BIOS.

"You cannot provide accidental-damage insurance for a user adjusting their BIOS."

Rightly so because adjusting the BIOS won't cause harm!

PS: if you are referring to damage caused by oveclocking (if perchance it's available in the BIOS), then this is a user-accessible feature. As such, it'd be covered under warranty.

If a manufacturer played hardball and tried to dishonor the warranty then they wouldn't stand a chance against most consumer legislation in most parts of the world. They'd be toast where I am, not only would they have to honor the warranty but they'd be fined in the process.

Perhaps you're in a part of the US where consumer legislation is essentially nonexistent then things might be different. (The US is known worldwide for having the worst consumer legislation in the Western world.)