From Champagne study guide "Grape acidity usually remains markedly high - an important attribute for sparkling wines"...I know and expect high acid in sparklers, but why? It seems that there may be a connection to the winemaking itself. I.e. the lower Ph is necessary to allow the wine to successfully/predictably go through secondary fermentation. With the various levels of sweetness it would seem high acid is more important with higher levels of RS, and less so toward the dry end. I can also see a potential connection between high acid and the ageability of the wine. What problems occur at higher Ph levels? Purely balance/structure, or is winemaking effected? Maybe all of the above and a happy coincidence that low Ph benefits every phase including consumption.
microbial spoilage becomes more of a concern at higher pH. This is especially important in sparkling base wines because they have much lower levels of added SO2. Getting a second ferment going with high levels of free SO2 is not fun so keeping a low pH wine cold and under inert gas are good tools to avoid unwanted oxidation or microbial issues.
I don't have a definitive answer, but I do have a couple of pieces. At high pH levels you've got a few things coming into play:
1) You're right that a high pH environment means its easier for undesirable critters to grow (which is why some wine producers will acidify their musts before fermentation, then deacidify right before bottling). When you've got a wine that's going through multiple fermentation processes, you want it as stable as possible. This principle translates into the aging process as well.
2) A high pH environment means that the effectiveness of your SO2 is significantly reduced. At a low pH, you can have enough SO2 to ensure that you have a clean primary and secondary ferment and not have to worry about your wine smelling like sulfur. At higher pH levels you run the risk of having to add so much SO2 that you screw up something along the way.
As far as acid and RS goes, that's more of a preference than anything else ... if you ramped up acidity with sugar levels, it would tend to mute the sugar ... sometimes you just want to have something sweet.
You definitely touched on it but acidity is a huge key in keeping wines fresh especially with regards to ageabilty. As the minimum requirement for Champagne is a lot longer for most styles of white wine, having high acidity in the base wines helps keep a fresh product during lees aging (and in many cases extended lees and of course bottle aging). I think you understand this concept but I figured it couldn’t hurt.
Secondary fermentation also provides alcohol in addition to S02, C02, and heat. The higher acidity is often contributed to early picking of the grapes-to retain acidity and have a lean, austere base wine of low abv-10-11% so that extra point of abv can be added during the second ferm. Any issues with RS can be regulated during dosage to balance the acidity.