A google union, or any tech union, can arrange whatever compensation scale it wants. Every union is a democratic organization, structured however its members want it to be structured.
There is no reason a tech union would have a strict seniority-based pay scale.
A tech union could do things like insist on better work/life balance, a seat at the board representing worker interests, you name it. A google-specific union could insist on bringing the 20% "work on your own thing" scheme back, for example.
Access to highly-skilled negotiators for your next review/salary negotiation.
No "crunch time", or at least limit it and require reasonable levels of overtime for it.
Protection for various kinds of paid leave.
Protection from being fired for political reasons—and, more generally, access to highly-competent employment lawyers in the case of legal conflicts between you and the company.
There is no reason a tech union would have a strict seniority-based pay scale.
A tech union could do things like insist on better work/life balance, a seat at the board representing worker interests, you name it. A google-specific union could insist on bringing the 20% "work on your own thing" scheme back, for example.