In addition to all of that, since your comment is spot on:
When people claim some variety is more conservative than another variety, they tend to cherry pick a lot. It’s easy, for example, to look at rhoticity and claim “American English” is more conservative, or to look at the cot/caught merge and claim British English is more conservative. But neither claim is accurate or meaningful; and when you try to look at the big picture, you notice changes everywhere.
To complicate it further, neither “British English” nor “American English” refer to any actual variety. Those are only umbrella terms; they boil down to “English, arbitrarily restricted to people who live in the territory controlled by that specific government”. And the actual varieties that they speak might keep or change completely different features.
In addition to all of that, since your comment is spot on:
When people claim some variety is more conservative than another variety, they tend to cherry pick a lot. It’s easy, for example, to look at rhoticity and claim “American English” is more conservative, or to look at the cot/caught merge and claim British English is more conservative. But neither claim is accurate or meaningful; and when you try to look at the big picture, you notice changes everywhere.
To complicate it further, neither “British English” nor “American English” refer to any actual variety. Those are only umbrella terms; they boil down to “English, arbitrarily restricted to people who live in the territory controlled by that specific government”. And the actual varieties that they speak might keep or change completely different features.