Aegis8: you really can't take any indication of ones popularity based on protest figures nor compare the protest figures directly with total population.
A: 23,2% of Russian population are under 19 years old. Also it is highly unlikely that minors would represent any significant volume in the protest, and it also means that this 23% must be deducted from the total population when looking at voters. eg Moscow with 12,5M inhabitants leaves you with 9,5M adults.
B: the median age of protesters were 31(
SOURCE ). As Russias median age in total is 39 (and much higher when deducting the minors unlikely to take part in any protests), leaves you with heavy bias amongst young people in russia favoring Navalnyi.
C: 18-44 year olds represent 34% of total population (eg moscow then 4,25M). Navalnyis support is at its highest in this age group. So if you take that 50000, correct it by, say 10% to leave older and younger out, leaves you with 45000 people. Which in essence means that roughly 10% of the age group participated. I consider that pretty significant as this is the biggest age group in Russia, mobilizing 10% of the people on the streets.
D: there is never, ever a full 100% participation in rallies/protests/whatever. It is always the minority that participates. Even if we would be talking about 10% of the supporters pariticipating (which would be very high), there would be other 450000 silent supporters. That would make a very high percentage in votes amongst adults.
E: Demographics is one thing, but the leader(s) of Russia are tangling to power and s
omehow people not aligning with Putin seem to have accidents and/or end in jail. Not only the poisoning of Navalnyi, but remembering Litvinenko, the Skripalis, especially Viktor
Yushchenko (suddenly got poisoned while running for Ukrainian presidency against Putins favorite), Politkovskaya, Vladimir Kara-Murza, Boris Nemtsov... all have met their fate in strange circumstances. This means that one would not participate in these types of rallies very easily, still enhancing the fact that the support for Navalnyi is greater than in your calculations.
F: If Navalnyi wouldn't be of any concern to Putin, why on earth would there be poisonings with a very rare poison by FSB staff? Fear of losing power has been the biggest driver for Putin during his entire leadership. Need to win election: go to Chechnya. Got someone you don't like in charge of ex-soviet countries: go to Georgia. Figures going down again: take Crimea. Someone challenging you: go Novichok (or any other military grade poison of your choice). Business world getting too much power? charge them with fraud/tax evasion/something else and send them to jail. Running out of presidential seasons? no problem, change positions with your muppet (Medvedev) for one season and run a constitutional change to start counting your seasons from zero. The more government denies it all, the more one can be assured that Navalnyi is concidered a threat by the authorities.