To be honest, I don't think it will be so disastrous!!!
There are MANY negatives which I hate, and speaking as an RS player since 2005, I would hate the old basics to change, much more than I hated SoF. I have only ever achieved 1 level 99 and that took me absoloutely ages. How people have the time to get 10+ level 99s is beyond me.
Many people have LIVES, and so the only way for players who want to play in a competitive way to gain a few 99s, while actually meeting real life commitments, was to bot. Many people agree that botting has been extremely detrimental to the game.
By making levelling easier, much more people will be less inclined to bot and actually play the game properly and reach higher levels manually.
Although I like the old "classic" feel of the runescape combat system, I think it DOES need a change. Combat to be honest is pretty boring in runescape. Auto-hits all the time, and just watching the outcome of a match boil down basically to pure luck was extremely frustrating. Adding at least a bit of dynamic combat would be a good thing in my opinon, as well as cutting down months of grinding constantly to get level 90+ in some skills and will increase the player base.
Overall, I mildly support the update, but I do think that Jagex should always make clear to us that they will be prepared to reverse the update in case it really does turn out to be absoutely terrible, which they would do if it completely kills their game and they have no more players left.