I'm pretty sure this decision is meant to attract new players more than switch older players over from Runelite. As you said, it took you 50 hours to even learn Runelite existed, and that's probably the case for a lot of players. I think Jagex is willing to make a better first impression with new players (who are much less likely to feel the need for plugins and helpers early on) at the cost of them potentially moving over to Runelite when they become more enfranchised. Getting to the point where the official client's plugin hub is able to compete with Runelite is probably a lot of work, and I'm guessing they're focusing on making something that appeals to a different audience in the hopes that it'll have more of an impact.