I wrote "depending on how it's embedded ingame".
Carribean! was based on a mod. Nw was based on a mod. VC was based on a mod. Now look at the success mercs and crpg had. If you understand capitalism you know that it's only a matter of time until taleword brings up a mercs-like multiplayer. Most likely a bannerlord dlc published when a new generation of m&b players gets tired of singleplayer.
@weapon customization: If they include weapon customization in mp, the questionis how. You won't craft your own weapons live every time you join a server. That would waste time and processing power. Having this in mind i see two options: Either your set of crafted things i saved on the server. So you join a server with your uid and get access to your stuff. Even if taleworlds doesnt provide such a service in bannerlord you wont need to be a great modder to host such an server yourself (bc weapon customization tool). The other option is that your set of crafted things is saved with your uid so you can use it on every server, needing no mods. That's pretty much mercs without the money-system.
No, it doesn't matter how a weapon customisation system would be embedded in the game, there's no way such a feature would make a mod like Mercenaries obsolote. Just bloody think about it.