Back in the early days, when the "Dungeons" part of D&D defined much of the gameplay, giving expo primarily based on Gold Piece value made sense. Even if you think it doesn't make sense, it's understandable - it was an easy short cut that kept things simple. And just think of the difficulty in removing that heavy hoard of coin.
Later, when "the wilderness" was initially introduced, much was hand waved and adventuring was still primarily in dungeon environments. Conducting home invasions of dungeon denizens was still the primary source of income and expo.
At some point the "sandbox" became a thing, even if it wasn't called such at the time. Exploration and the wilderness came to the forefront but the method of awarding expo did not change. At 10 gold coins to the pound, how did anyone travel with money? (gems, of course)
Sandbox play damn near requires a rethinking of the method to award expo. Do you give points for each 6 mile hex explored? Double the xp from monster kills? Gold and the like is much rarer in the wilds.
By it's very nature, sci-fi roleplaying is very sandboxie - no one plays scifi on a single world, they want the universe. Can credits in lieu of gold really be sufficient as a source of expo? Doesn't this hamper the viability of a Space Marine themed campaign, where killing xenomorphs doesn't lead to the recovery of credits?
What, if any, alternate methods of xp awarding do you use? Do you change methods depending on the type of adventure? Is gold = XP an OSR sacred cow?