I'm a DM. I improvise more than I follow adventures. I make on-the-fly rulings all the time. I try to be fair, be on the players "side," while allowing risk of failure. I make clear rules at the beginning of a campaign on what classes, races and other options are available, and I'm not afraid to straight up tell a player that his character idea isn't going to work in my campaign, even if it seems to follow those rules. I like wild mages.
And I am not satisfied with the lack of direction regarding "...DM can..." in the Wild Mage.
I see myself as a fairly similar DM, but I
do like this case, assuming it really is a "DM can" case.
It makes every group where someone chooses to play a Wild Mage, stop for a couple of minutes and talk it over about what to expect, then
agree on the level of randomness that this PC should bring into the game.
If the PHB said a fixed amount, certainly having a default would make it easier in a certain way, but would probably just split the gaming group between those who are
in favor vs those who are
against the character being in the game. Leaving it totally open for the group means there's a much higher chance that a compromise can be found so that the PC is accepted by everyone.
Most importantly, note that "DM can" actually includes the option of
never rolling on the wild magic table!! So if no one wants those random effects,
you can play a Wild Mage that never causes any havoc to the game. And the character is still balanced, because those subclass benefits are quite reasonable even without the random effects. Those random effects include both beneficial and negative consequences, so even if the overall unpredictability is for someone a price to pay, the consequences may even out (although extreme consequences may be disruptive both when positive and negative).
And the Wild Surge table is *always, always* trite.
There is no solution to this that would fit in a PHB.
It's meant to be just a series of examples. After a few adventures, you're supposed to make up your own, e.g. pick the first book you see, open a random page, point your finger and that's what happens (be it a spell, monster, or whatever).