Depends on which legendary path you want to follow. You have the Hongmoon legendary at later 50 and the legendary that will come with Naryu Lab, these two are completely different legendary. If you go Hongmoon legendary path then yes your pirate wep evolves into it later, if you go Naryu Lab legendary then its completely separate from Hongmoon weapon path.
IMO I’m most likely going with Hongmoon legendary since NA team announced on their stream we’re not getting the Dragon Tiger (TW) version and we’re getting Storm (KR) version. Dragon Tiger version is amzingly good, but the KR Storm version is meh and its not as good once we get Hongmoon legendary.
If we get the KR legendary it’s worse than True Siren and can’t be upgraded until we get the quest to make it into the red version, which is barely better and still worse than Pirate.
True Pirate can be upgraded into True Gale when 50 hits which skips Devil/Awakened Devil/True Devil/Awakened Gale. True Devil and Awakened Gale require Silverfrost Evolve Stones to upgrade and those are going to be expensive AF so if we get the crap KR legendary True pirate will save you a lot of money.
Upgrading to True Pirate will be the cheapest in the long-run. If at all possible, I suggest trying your best to get Blade And Soul Gold at least to Awakened Pirate before level 50 hits. Getting to Awakened Pirate will guarantee you to True Pirate and then on to the level 50 weapons.
As for legendaries, in the other versions, they are largely considered P2W items because of how expensive they are to upgrade. In Taiwan, you can upgrade your legendaries without paying a cent but it would take you months or perhaps even years. I honestly have no idea what the costs will be like for the NA legendaries. Expensive I’m sure, but perhaps more feasible to actually get compared to the other versions. We won’t know until they’re out.