There are plenty of unions (some examples up thread) that do variable pay. If you're arguing against a specific kind of collective bargaining agreement, then just say that, and be aware that it's a far more oblique criticism of unions, especially in this specific context of Google.

Also, a salary agreement between a single individual employee and a huge corporation has by definition a huge power imbalance. Unions reduce that power imbalance. You can argue which is best (full negotiation freedom with massive power imbalance vs. collective bargaining with or without variable/performance pay and a much smaller but still non-trivial power imbalance), but there's no point in just arguing for one skew and ignoring the legitimate arguments for the other.

As I said in my original post, the power imbalance depends hugely on how marketable a person's skills are. A company being much bigger and richer than an individual isn't a weakness of that individual's negotiating position if the company really needs that developer and that developer doesn't need that job and both parties know it, it's a strength. There are people that Google would find incredibly difficult to replace, and few of them would find it more difficult to find other well paid jobs. A third party which is less interested than both original parties in prioritising the interests of the highest performers over workers 1/10 as productive is almost certain not to negotiate as effectively on the highest performers' behalf, whether the agreement involves a range or a fixed wage. Collective bargaining makes workers considered individually dispensable by the company indispensable by threatening to collectively withdraw labour. That doesn't help those who are already indispensable and know about it, and typical collective bargaining goals (reduce pay inequality, increase transparency, reduce or eliminate scope for individual wage negotiation, increase the amount the company spends on less productive employees) actively hinder them in getting the pay rises they could ask for.

As I pointed out a very obvious situation in which power imbalances ensure even the best performers would benefit from collective wage bargaining from the very beginning, I'm pretty confident I am not the person in this subthread arguing for just one skew. But I am equally adamant that there are people Google considers [near] indispensable, and most of them are plenty employable enough for wage bargaining to be skewed in their favour, an imbalance not improved by people who want others to see pay rises first taking over negotiations.

A wage schedule that increases minimum salary by x% is more restrictive, but benefits everyone and hurts no one.

Your "by definition" is incorrect. There's no law that requires a collective bargaining agreement to enforce pay maximums (or in fact anything about pay at all!). So please don't tell me that I don't know what I'm talking about.