I don't believe there is an authoritative statement from any legislators who support court expansion at this point on the issue of retaliation. But articles that cover this area usually address this point.
During the Democratic primaries Pete Buttigieg put forth a proposal for expanding the court that would preempt retaliation by explicitly balancing the court in its political leaning. After enforcing a "fair" court the hope would be further expansion by Republicans would be seen as illegitimate.
Buttigieg proposed expanding the high court to 15, but not simply by allowing the sitting president to amplify his or her ideology on the bench. His plan would have five justices preferred by Democrats and five by Republicans. Those 10 justices then would select their other five colleagues.
In the Atlantic, Aaron Belkin, a political-science professor at San Francisco State University and the executive director of the think tank the Palm Center asserted that it didn't matter if Republicans eventually retaliated because the only other option was to retain a permanent conservatives majority.
This is perhaps the No.1 concern that’s been voiced, but it doesn’t stand up to scrutiny. A couple problems with this: The first thing is that the Court has already been stolen. If your wallet is stolen, you don’t forgo efforts to recover it just because it might be stolen again. It would probably take a generation—25 or 30 years—for the Democrats to get the majority on the Supreme Court back. If the Republicans steal the court, then the Democrats un-steal it. And if the Republicans steal it again, then the Democrats un-steal it again. It’s much better to have that zigzag than to just have unilateral surrender.
In New Republic; David Faris an associate professor of political science at Roosevelt University also used a similar argument.
So what? At least that would produce courts that are more responsive to public opinion rather than serving as the last redoubt for a long-expired political majority.
He also addressed some of the other points you raise about resolving the lean of the senate toward smaller states, where the national majority party end up as the minority in the Senate.
And if Democrats use their newfound power to address other features of the electoral system that give Republicans an asymmetrical advantage in national elections—by enacting statehood to D.C. and Puerto Rico, passing a new voting rights act, and implementing national ranked-choice voting, for instance—it could be many years before the GOP controls Congress and the presidency at the same time, for the very simple reason that Republicans are likely to continue to be unpopular and will rarely win at all in a reformed electoral system.
