Well unfortunately that's the way the market works - it wouldn't...

Well unfortunately that's the way the market works - it wouldn't be legal for Rio, or any company, to retroactively give some sort of benefit to a subset of holders just because they've been on the register for x number of years. Shares in the same class rank equally, whether they were bought a day ago or a decade ago. That's how the world is, and it's how its pretty much always been.

"NOT GIVING THE ORIGINAL HOLDERS A CHANCE TO BENEFIT FROM THIS ENTITLEMENT even though there stake in the company has been reduced by about 15%."

You get the same rights given to you, whether the date is two weeks from now or was yesterday. You get the same dilution if you don't take up, and you get the same number of rights that you can take up or sell at your discretion. Other holders, who may be long-term shareholders, may choose to sell before the ex-rights date. This is their private decision, and importantly it does not change the entitlement you get, the value of that entitlement or the dilution to you if you do not take up.