The main issue is that to replace all advertising revenue across the board, everybody would need to pitch in something like $150/month. I like paying for things to get rid of ads, and I'm not even sure I'd make that commitment.
(There's also the fact that the utility of ads is fundamentally bounded by what they can make you spend. By their nature, the amount someone is willing to spend to remove ads and the amount of money ads can possibly be making on the person are probably held more tightly than you might expect, due to the underlying third-factor of correlation with the amount of money the person in question can spend.)
The unfortunate thing is we're already paying that money, since the money spent by companies on ads comes out of the revenue we give them for goods and services.
Hence we can afford to support things through Patreon, Flattr, etc. instead of through ads, if the existing money were shuffled around.
We probably can't do it as well as through ads, but to free up that money the sellers of goods and services would have to stop charging us for their ads, and there's no incentive for them to do that :(
There is a possible mechanism that could free up that money: without ads sellers can't rely on popularity to sell their products and have to rely more on underselling the competition.
I can't find this anymore, but did find an estimate that internet ad industry did $60 billion last year. Assuming 1 billion users ( I know more people have internet but lots probably don't interact with ad-ridden sites), you're talking $60/year. Which is pretty good overall.