So for many games (especially XP for GP games) you get the old problem. Players run out of things to buy and start investing in things...that earn more money. This they then invest into even more things and/or level up through their merchant empire.
So the first step XP for GP games do is say "XP only comes for treasure recovered from dungeons or the like". Personally I think it cuts out the opportunity to have a merchant expedition to far off lands as an adventure tool (load up on cargo then fight pirates, and storms, and forage for food and water on cyclops infested islands to reach the far off port), but you can always make exceptions for such daring adventures (until you've charted a route and it becomes routine).
Even without XP though, you still end up getting a tonne of money that ends up making more money. The need to eat and buy supplies is a powerful motivator to adventure and take great risks. If you'll starve without money, the temptation to try and steal the idol's ruby eyes despite the warning of a curse becomes much more important. If you can try again next week and return to sleep on your featherbed in your castle...well... maybe you'll let someone else make the save or die check.
Both Al-Qadim and Lamentations of the Flame Princess address business ventures in a simpler way: They make them mathematically suck and eventually lead to ruin. That works functionally, it keeps adventurers adventuring but it breaks as world building for me. I mean, saving up for retirement is a motivator for adventuring. Get that big haul so you can retire. Invest in a business and live the high life.
Conan does not invest in mutual funds with a healthy rate of return.
Conan also shows the solution. Make investment easy and logical. Investing in a business venture should be likely to produce wealth, it is why people have businesses after all. Its both the why and the how of the innkeeper keeping his inn afloat. But go a step further from only getting XP for gold from adventuring: Adventurers lose XP for gold earned through normal business ventures. I'd still leave room for the daring adventure to first open a new trade route, but not for regular routes (and even still that is a personal preference).
But when they start running businesses a warrior's mighty thews begin to weaken with their day to day living in civilization. Jewel Thieves begin to dull their reflexes after a few weeks of safe transactional business where the worst danger you face is an employee skimming off the top. Wizards stop pontificating on the mysteries of the universe and start focusing on accountancy and taxes. They go soft.
I would suggest this as a form of retirement, they don't lose levels but they do lose XP which makes it harder to hit the next level. This means that retiring upon hitting max or name level makes sense (you have nothing to lose really since you are max level already). But a 3rd level warrior investing in a mine? The 3000 gold pieces he earned this year simply pushed him far from ever hitting 4th level.
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