Corbynism for a decade? It no longer sounds ridiculous.

John McDonnell sets out the party leadership’s best case scenario in an interview with Fabian Review, saying: “I want to get Labour elected in 2020. I’d like to serve a term as Chancellor so we get the economy back on the road.” Although Jeremy Corbyn has not confirmed his thinking, he is believed to favour a similar timetable. By 2025, Corbyn will be 75, just five years younger than Winston Churchill, the oldest Prime Minister of the 20th Century, who was 80 when he resigned in 1955, and 10 years younger than William Gladstone, who was 85 at the end of his political career.

Clive Lewis, who is believed to be the most likely successor from the Corbynite wing of the party, will likely be brought into the shadow cabinet if, as expected, the leader conducts a reshuffle in the new year.

Although Momentum, the organisation set up as the continuation of Corbyn’s leadership campaign, has found itself under fire in recent weeks, Momentum staffers believe that the increasing ferocity of criticism directed at them is because “they cannot attack Jeremy directly”. Michael Dugher, the outspoken shadow culture secretary, branded the group "a mob" in an interview with The House magazine, while the group has been stung by a complaint about data protection.

They are privately unfazed by Dugher’s attack and are confident that the information commissioner will exonerate them.

Indeed, Corbyn’s allies have a spring in their step following a series of triumphs in three “proxy wars” with the leader’s internal opponents. The first, over the composition of the leader’s office, has been decisively won by Corbyn. Andrew Fisher, the leader’s advisor, who came under fire for tweets appearing to endorse Class War against Emily Benn, the Labour candidate in the safe Tory seat of Croydon South. The complaint was struck down by the party’s ruling NEC. The second, over Corbyn’s attendance at the Stop the War Christmas party, has also seen the leader’s critics disappointed. Although Stop the War has come under fire as a front group for the SWP and a variety of smaller parties on the far left, with both Caroline Lucas and Peter Tatchell criticizing the group, Corbyn will still attend the Christmas bash, despite urgings to avoid it from the PLP.

But the third is perhaps the most significant. Although both the leadership and MPs are playing down the divide over airstrikes in Syria, because, in the words of one aide “they [the 66 MPs who voted for airstrikes] don’t want to be shouted at by their members and we don’t want to look divided”, all sides are aware that many more than the 66 MPs who did vote for airstrikes were convinced on the case for extending British bombing against Isis from Iraq into Syria, but pulled back due to pressure from their constituency parties.

That raises the difficulty of dethroning Corbyn still further. Although legal experts are divided over whether the Islington North MP would be forced to seek nominations again in the event of a challenge, with Jolyon Maugham, the respected barrister and former Labour donor, arguing that he would not, and GRM Law, who have advised the party on numerous cases, telling MPs that he would, most now accept that Corbyn would be able to pull together the 35 MPs he would need to stand again.

“It’s unthinkable that if 40, 50 MPs won’t vote with the consciences on what to do about Isis, that they would keep Jeremy [off the ballot],” says one party grandee. That means that any attempt to remove Corbyn would have to be backed by a candidate capable of defeating him among the party membership – no such candidate exists in the parliamentary Labour party.

A combination of what one shadow cabinet minister calls the activists' “sense of fair play” and what you might call “exitism” –a steady stream of anti-Corbyn members leaving the party – mean that Corbyn would easily be re-elected by the pary membership. "Jeremy is the strongest he's ever been," says one ally.

Corbyn will celebrate his first 100 days in post in ten days time, with one veteran from the leadership campaign observing: “If you look at [us], the only wounds that have made a mark have been self-inflicted”. They believe that if the leadership can cut out its “unforced errors”, they should face no serious obstacles between now and 2020.

Speaking to Fabian Review, McDonnell shares that criticism, saying that the party “has got to be quicker, and we’ve got to be sharper” in its all-round approach.

As for Corbyn’s internal opponents, depression is setting in. Although it is widely believed that Dave Prentis, the incumbent general secretary of Unison and Corbyn critic, will see of the leftwing challenge of Heather Wakefield, few expect the “big three” affiliated general secretaries to be able to force out the leader, as Len McCluskey himself faces a tricky struggle to secure his re-election in 2018, while Tim Roache, the newly-elected general secretary of the GMB, is believed to be focusing on internal issues over the coming years.

Small wonder that the party’s Corbynsceptics are increasingly despondent. One peer, speaking to the New Statesman, said “I now think it might be terminal.” Another was more blunt, saying only: “I’m going to die before we see the Tories out again.”