I think the current issue preventing players from logging in to both games simultaneously on one account is that the RS3 and OSRS game servers are managed by the same meta-server. The meta-server manages things like counting how many players are on the same world and saving permanent data like XP and bank contents. This means that if the meta-server sees you logged in on RS3 World 12, and you try to also log into OSRS World 492, the meta-server won’t let you. This is also why Jagex was literally unable to let you play on Leagues and main at the same time. It’s not that they want you to pay for more membership to play two separate accounts at once, it’s that their server architecture can’t handle logging the same account into two servers at once. While splitting the meta-server into two, one for managing RS3 worlds and one for managing OSRS, would allow you to log into both games at once on the same account, the process of splitting the servers would probably break a ton of things and might risk account data loss.
TLDR it’s not Jagex saying “haha pay me double membership” it’s Jagex saying “we’ve been using the same back-end server management code since RuneScape 2 launched in 2004 and we literally can’t enable you to play the same account on both games.”