Well, some BTC transactions are already replay protected.
Of course they are. If you have one splitted coin, then everything it touches, is also splitted correctly. It is enough if a miner uses a coin from any newly minted block. Then, everything touched by such coin is always splitted, even if there is no replay protection.
this trend toward transaction separation will be enhanced among users over time
Of course. However, if some coins will remain unsplitted, then they could be replayed at any time later in the future, unless some chain will introduce new consensus rules, to block it. All altcoiners have an incentive to keep things replayable for as long as possible. Because of newly minted coins, they cannot keep coins from splitting forever, but well: they have no reason to make coins harder to replay, because they can get more coins, by depositing splitted coins to exchanges, and withdrawing unsplitted ones, and replaying it on their side, as long as there are enough supply of yet unsplitted coins.
