I don't think it is necessary to make (some) privileges permanent. They are, except for users with very low reputation, rather difficult to lose, as losing reputation is not that easy, both in theory and in practise (as on this site).
I know you lost your privileges, and I see that as an exception (from my observation visiting the site, not from statistics).
First of all let me say that that when the other members of a site let a user lose reputation, that doesn't mean a user is a bad person, or that that user doens't have valid questions. It is primarily a means of telling that user that something is wrong with their contribution to this site, either with regards to the content of their questions/answers, or by their use of the site.
Reputation levels for priviliges and reputation change
If you look at the help page the privilege changes are at 5, 10, 15, 20, 50, 75 100, 125, 250, 300, 500, 1000, 2000, 2500, 3000, 5000, 10000, 20000 reputation points. The "normal" reputation gain is by upvoted question (+5), upvoted answer (+10), accepted answer (+15) and the "normal" reputation loss is through downvoted question (-2), downvoted answer (-2).¹
So normally if a question is dubious, and the same number people vote as vote down, the OP still comes out ahead 3 points for every up-down pair of voters.
That however is not the whole picture. If someone upvoted your question and decides to down vote it, the nett difference is 7 points. The system only allows a "voter" to do that if the question has changed.²
In practise, if you don't edit your question except for cosmetics, voters on this site don't retract their upvote or change it to a downvote.
As the reputation level sequence shows, the more reputation you have the less likely it is that you loose a privilege (as the gaps between there where your rep can "move about" becomes larger).
Users who answer a question are more than average likely to have read and upvote that question. And since a reworked question, invalidates their answer and the effort they put into that—with the additional chance of a retracted votes or downvotes on their answer because it doesn't answer the question in the post—they are more likely to retract/change their upvote on the question than others. So this compounds the effect a radical change of a question will have.
How to get and keep your upvotes
If a new users concentrates on his/her questions to be clear and concise, by stating both the context of the larger goal and the immediate problem, there is less necessity of correcting the question and that minimizes the chances of the -5 or -7 rep change with retracted or changed upvotes.
If there are already a response that answers the question, even if that doesn't solve the real problem, the OP should not further change the questions meaning, even if they realise it did not solve their problem. Certainly not if that question was upvoted and thus someone indicated that the question, as it was at that time, was valuable. If the OP realises there is a different problem, than there is room for a different question (with more chances for upvotes). To prevent closure as duplicate the OP should refer to the other question and clearly state what is the difference and for the second question give some extra thought about why the first didn't answer the real problem, so there is no need for a third related question.
If a question gets downvoted before any answers appear, an OP can consider to close it, and think about how to improve it (or search some more for duplicates), often, but unfortunately not always, comments might be of help.
Facit
Following the above steps it is relatively easy to prevent retracted/changed votes and get out of the danger zone of loosing a privilege. Normal downvotes' (not reversed upvotes) effect on rep (-2) are so small, that this is far less likely to be a problem.
I see the incentive, of trying to keep privileges, as an extra motivation, beyond reputation alone, to make and keep questions (and answers) clear and relevant and worthy of other peoples upvote. And with that in mind, I see no reason to make (some) privileges permanent.
¹ There are other reasons for rep gain and loss, but involving them do not add to the argument
² The same holds for downvoting a previously upvoted answer, the difference there is 12 rep points
