The math doesn't work like that in a system heavily skewed by bend points. The lower earners should be paying taxes on an even higher percentage than the upper earners, because their benefit is much higher per dollar contributed. Their post tax contributions are a much smaller part of their benefit checks. In a system with bend points, there is no generally applicable "average", your payback percentage is widely variable depending on where you land on the bend points. And it works in the inverse, where the lower your benefit, the greater the percentage that should be taxable. If your litmus test for taxability is just giving credit for the portion of benefits that are return of post tax contributions, then there is only one point on the bend points that lines up with 85%, with some paying too much and almost everyone else paying too little.
There is no "growth" in SS. There is inflation adjustment and that's it. The employer & employee contributions cited on SSA.gov are in nominal dollars. The post tax, employee contribution dollars plus some type of time value return would have been tax free had it been put in a Roth, or at least taxed at LTCG rates if not. The maximum justifiable mathematically "fair" way to assess tax would be 50% of benefits (the portion attributable to employer contributions) as ordinary income. The other 50% minus the nominal employee after tax contributions (all of them, not just best 35) counted as LTCG. That is an apples:apples comparison with other options. No one should be paying ordinary income tax rates on 85% of their SS benefit.
But to get even further into the weeds, the variable percentage of SS subject to tax is then run through another very progressive system - the federal tax brackets. What is the result of one progressive algorithm run against another progressive algorithm ? Progressive^2 ? And the money collected is put back into the trust fund. .
I suspect taxing SS benefits was just another money grab from upper earners to shore up the system in a politically palatable way. That would match with the budget concerns that drove the action in the first place. No one was thinking to make the system "more fair", because the action taken was the opposite of fairness. Anyone that believes the rhetoric in support of any .gov action is the total real story hasn't been paying attention for the last 50 years.