This is true to some extent. I think though assuming it's a good quality brand, and you have an idea of your necessary firmness, you can reliably pick a mattress by trying it out in a showroom for a bit. A pocket sprung mattress is just springs with a layer of soft stuff on top, after all, it is not some special magic sauce. JR likes to say all over its website you can't trust showrooms, but they have an obvious biased interest to convince people to buy blind on the internet. But 99% of people choose their mattress by trying it out in person with generally good results. I mean, this is what I've done over the last 20 years and every mattress I tried out was perfect for me long term. In fact in my own experience buying blind on the internet has produced the most trouble, I've already had to return 2 mattresses now.
So probably the ideal thing is try it out in person first, and buy with a good return policy. I think lots of places do offer 30 day returns now. Ikea does 1 year returns (but you'll have to rent a van to return it yourself). W/ lockdowns you gotta just buy blind, which is what I did.
