From my point of view, as an aspiring designer and store owner, this is terrible news to me. For the last few years I have been able to squeak through with a quarterly earning target of just over $6500L. With careful uploading accompanied by testing out my uploads on the Beta grid as I could find a sandbox over there, I could slowly add things to my store inventory. The $300L a week stipend helped offset my premium fees. This quarterly target is now raised to $9000L and the stipend will not cover the difference.
So my suggestion, if you truly want to add value to the premium then raising the stipend from $300L to $500L a week is a good start. This would greatly assist and more importantly alleviate the incurred extra cost to the premium users. Further it would put just a tiny bit more disposable income in the hands of users and by extension help to stimulate spending in second life as is being intended by these changes.
The alternative for me right now .. and I am certain I am not alone .. is to close my in-world store, sell my land and for the first time in I believe 7 years return to being a non-premium subscriber. This will greatly temper my participation in second life particularly as I do take advantage of the premium access sandboxes from time to time to prepare things for my store. In short, this update to the premium fees may well price me out of premium.
Edit: Now I may be making an assumption that I can not retain the rate I have been paying for the last 7 years, and that it is new subscribers who will have the updated fee. A clarification of this with a clear declaration of weather or not the current fee will be grandfathered and renewed until the subscription is ended. The wording in the announcement about the fees is a bit ambiguous to me.