But as you said, its a sandbox game.
It isn't about PvP, that isn't the focus of the game, because as a sandbox voxel game it doesn't really have a focus beyond building things. Fighting is something you can do, its something you occasionally need to do, but it isn't the main reason the game exists.
Especially when it comes to multiplayer, forcing someone else to play the game in a way they don't want to is *BAD*. If they want to fight, thats great, get in a big old fight and see who can build ships better and faster, knock yourselves out. If they don't want to fight you, then back off and let them do their thing.
As has been stated in the thread already, there is no real reward to victory, and there is a huge cost to defeat. If you are willing to risk that, that is a decision you are allowed to make for yourself and yourself only. Forcing someone else to play the way you want them to when they don't want it is just you being a jackass.
After reading everything I think you may have misunderstood what the OP wants. I hope if I get it wrong he will let me know. But the way I understand it, he doesn't want to "Force" anyone to fight. He wants there to be risk/reward to the fight. Something for the winner to gain that makes the blocks they may have lost fighting worth the loss.
As an example, I myself build my ships with the idea that I will pilot them, use fleets and run my own empire of sorts. Using economic, political, and militaristic means to run everything effectively. There are a few issues with this however and they come into play with other play styles as well.
First off, there is currently no incentive to expand. Weather its building more fleets, more stations or leaving your home system. There is no reward for it other then the fact you built something and it looks good. This is not enough to hold myself and many other players I know. There is a reason the MC server I used to be staff on is dead. Its because there was little to do in the sandbox other then build a bigger sand castle.
There is no reward for a fight. Only risk. If I have to risk something, be it resources, time, fleets, or anything else, I want to have some kind of benefit from taking that risk. I could salvage whats left of the other players ship/fleet. But why? Currently mining is more effective then bothering. At least IMO.
There is no real economy. Why bother trading when you can get everything you need with ease? Sure you CAN trade. But you get little from it. There is no specialization, and resource distribution allows factions to be 100% self sufficient at all times. Add to that the fact that there is no reason to expend resources fighting or expanding other then for the sake of doing it, and you have killed any kind of economy that might have appeared on a server.
The real issue is that we have a sandbox, we have toy's in the sandbox but we don't have a rule set that lets us strategically chose how to use the toy's. We have the sandbox, but what we lack is the "Game" that takes place in the sandbox. Now I don't expect any of the rules to be like "You must build a faction and expand and conquer everyone". But at the same time having a reward other then a look they fear your ships and hunker down in their invincible HB to never fight you again.
I would agree that there should be some kind of "Loss" for losing a faction war. There are many mechanics that could allow for it. Weather its gaining valuable territory, FP, or forcing a faction to disband for X amount of time, or something completely different. The point is, there is no point in just building things that have no purpose in the world. And although a player CAN assign a purpose to the things, if the game world doesn't support that purpose (like your fleets that can't do anything because players are all in their HB's) then the sandbox has failed. It gave you the sand to build with, it gave you the armies to command and the resources to spend. Then it forgot to give you something to do with it all.
I believe that starmade will eventually encourage conflict. By encourage I don't mean force. There is a BIG difference. to encourage it you just need adequate risk/reward. Forcing would be employing a rule set that actively punishes players for choosing an alternate path. I think most play styles should have their advantages and disadvantages. the advantages/disadvantages don't have to be equal ether. The Dev's can chose to encourage some styles more then others in order to make a more compelling experience for everyone.
When you say the game is not about PvP, I think you have missed something. The game mechanics are all in support of building combat ready ships. Why have combat ready ships if they don't serve much purpose?
/end rant
TL;DR:
I think encouraging PvP, Faction conflicts, and general reasons to have combat ships and fleets is a good thing. I think encouraging exploration and expansion while not forcing it and/or punishing players for alternate play styles is good for the game. I think what i jsut said sums up the OP's points. And I agree. Method however is debatable.