The value of homes is very well known and assessed annually in many provinces (some have weirdly become laggards). So no real problem there.
Any piece of art that is of any real value would have a provenance and it would be very well known what the value it was at any given time and at sale. If no one knows the artist or can determine the value it is very safe to say its value is nil.
But these types of things are found all the time in attics and basements. Art especially is moved around without sales records all the time, and jewelry even more-so.
Heck, I have things I bought myself that I have no idea what I paid for them.
But I'd sure be upset if I had to pay cap gains taxes on these things assume their prior value was zero.
On the other hand, cost basis in a house is not just the purchase price. Many improvements add to the cost basis, and good luck finding records to support that. Especially for a home owned by your parents since the 1970s.
That doesn't make it equitable to step-up on death; but it does make it very convenient.
I think the government now actually does keep tax records of buying and selling homes (became a bit of a question during the foreign buying debate) so going forward it's going to be no concern.