Your product tests exactly one hypothesis. Making needless assumptions just compounds your chance of failure before the real exam actually begins.
Disproof
Your problem statement has exactly 1 (one) thing that you need to fuck around and find out about, and it is explicitly defined as your product hypothesis
Consequences
You become that company known for torching millions to build something they could have found out wouldn't work/sell just by asking a few passerby
Causes
Usually, a combination of excitement and a damn-the-torpedoes attitude, exacerbated by people trying to look good
It's rare to be completely oblivious. Normally, the excitement to build runs away from you juuust long enough that when you come to your senses, sunk-cost fallacy kicks in, and "the only way out is through", etc.
Unfortunately, there is a very real fear of being disagreeable / being blamed for wasting time that somehow leads to people doubling down and wasting more time
Approaches
Look at your problem statement. If any assumptions can be tested without building the entire product first, it is your duty to validate those to the best of cost effectiveness
Take a good hard look at what remains and work out how screwed you’d be if none of your assumptions turned out to be the case, and decide if you’ve got the stones to roll the dice
That’s not to say don’t do it, as you’ll find out a lot by being in contact with reality. This might provide workable opportunities, but either have a plan or be prepared to think very fast and definitely have some budget
Practice...sigh...awareness. Have something built in to your way of working that structurally forces you to come up for air