I agree, but it's important that what qualifies as investment matters. Perhaps a function of comments, votes, submissions and time the account has been around.
As an aside, how would we distinguish between RMS posts and regular posts? Would the spammers simply revert to normal posts? I think if this is the case then perhaps we should reconsider the goal.
If the goal is to stop spam masquerading as RMS, then it seems to me that accounts should require more longevity or community contribution to post an RMS post than a regular post. As long as we define etiquette for posting an RMS post, then it should be feasible to use a combination of following that etiquette, and flagging posts that don't follow said etiquette.
If the issue is stopping spam, then perhaps that's separate to RMS posts and should be considered as a wider whole.