I am not quite sure how you got to your final point. If you are working on the assumption that prison is a cure for crime, think again.It's easy to go over the top in crying for harsh punishment, but we should note that these are very long sentences when compared with similar acts in the UK. Their attack came 5 months after a climate change protester threw a cake at the Mona Lisa in Paris. I don't think he's been charged.
The Countryside Alliance lot who stormed the floor of the Commons in 2004 got suspended sentences.
The Fathers for Justice who threw condoms full of flour at Tony Blair in the Chamber also in 2004 got three figure fines.
Cai Yuan and Juan Jun Xi who had a pillowfight on Tracey Emin's 'My Bed' in 1999 were arrested, but not charged.
Vladimir Umanets got two years for causing an estimated £200k worth of damage to a Rothko in the Tate in 2012. The JSO pair damaged the frame of 'Sunflowers', largest damage estimate £10k.
The suffragette (and future member of the BUF) Mary Richardson, put seven slashes in the Rokeby Venus in 1914 and got six months. That painting was repaired and is estimated to be worth £72.5 million today. (Who knows what it would be worth without the attack? I'd guess £72.5 million. Let's see what the JSO protestors get for attacking the protective glass of the same painting in November last year.
The two blokes charged with felling the Sycamore Gap Tree go on trial in December.
The only longer sentences I found for non violent protest were for the JSO activists who targeted the M25 convicted last year. They got 4 or 5 years each. Five years is only six months less than the average sentence for sexual offences. The average sentence for violence against the person is just over the 20 months that one of these two got and less than the other. Her sentence was approx twice the average sentence for weapons possession. https://www.statista.com/statistics/1100192/prison-sentence-length-in-england-and-wales-by-offence/
The suffragette lesson suggests that it is the reactionary response to these protests that will make them historic, not the protests themselves. Apart from one of them being Bryan Ferry's son, nobody remembers the Countryside Alliance lot. They were quickly forgotten in a way they wouldn't have been had they got a couple of years inside. Whereas the strong sentences handed to the rioters in the summer acted as a deterrant to those who may have considered getting involved in future trouble, it's likley to have the opposite effect on committed political activists. These sentences will not stop these protests. They will just create political martyrs for the movement.
Its a horrible situation. They are right about the urgency. They are right that the world isn't acting as it should be to save itself. You can understand the frustration at billions of us doing nothing that will inconvenience us. Rightly or wrongly, they acted radically according to their consciences, but we all know that after they've served their time in prison, they'll come out and nothing will have changed.
My understanding is prison is part deterrent (of questionable value - it deters me as a non-criminal, but....) and part punishment.
There is a wider question about sentences for different crimes, but this has always been the case.
Perhaps sentencing should be determined by a committee of readers of The Guardian. Or readers of the Daily Mail. Or both.
One thing is for certain. There will always be somebody who is unhappy about someone else's sentence. And most of us imagine we have a far better idea of how sentencing should work than the people running the system. I suspect we are wrong.