Yes exactly. You're telling me to make two separate orders and deliveries, multiplying the admin, postage, packaging, and delivery - up to several times 'trying something else'. Lots of waste.
I suggest trying both at once where there is a high chance of my guess at sizing being wrong, knowing I am limiting it to one return regardless of whether I keep one or none.
When you literally cannot buy a duplicate item without it being a different size from the one you already have, the chances that your best guess is going to fit are very small.
I do agree that people abuse the free shipping threshold by adding in items they have no intention of keeping. I don't know the best way around that. Personally I'd rather pay £4 than buy something knowing I'm going to have to have the faff of returning it.
I think Next must've had serious problems with tons of returns, as I had Next Unltd where you paid £20 for a year of free shipping and returns. The next year they had taken off the 'free returns' and you still had to pay a few quid. I think there was some penalty for returning an entire order, too. I didn't bother getting it again because their sizing and item information is so inconsistent/ non-existent, even a few returns a year would add up.
When I had the Next Unltd it was my first stop when looking for kids' clothes, my clothes, brands etc, so ordered a fair bit from them without looking elsewhere. I am more picky and shop around now!