Boundary changes are a coming, the membership is very supportive of Corbyn.
Not sure if you are a Labour member and understand the way in which selection occurs. Well I am (at least for now) and from my experience standing MPs are re-selected as of right unless there is a trigger ballot decision to the contrary. Only if that happens will there be a contest between the sitting MP and other potential candidates.
And the membership that selected the MP in the past will, almost certainly, be the same people who will reselect him or her in the future. Why, just because there is a different leader would they suddenly chose to kick out an MP they previously supported. And that is not-withstanding the new members.
Why because most of those new members are likely to be 'armchair activists' - happy to vote in a leadership election, but very unlikely to actually take part in anything else.
And I speak from experience - my local branch (like many others) has seen a (on paper) massive increase in membership. I think something like 30% or more increase. Yet talking to friends who have been the mainstay of the local party for years (councillors, election agent etc etc) how many of those new members have put themselves forward to deliver leaflets, to stand as candidates, to go out on the doorstep and canvass - effectively zero. How many even had the get up and go to attend the regular local branch meetings - exactly zero.
So the people who will be responsible for the success or otherwise of a sitting MP in a trigger ballot will be the same people (almost entirely) who were previously.
Try it this way - in the mid 1990s there was a similar massive surge in members - largely Blairite centrists, who would never have joined in the 1970s and 80s. Did these new members kick out non Blairites, such as Corbyn himself under the same rules - no they didn't. He continued to be supported by his consistency, just as (I have little doubt) Hilary Benn will, and Liz Kendal, and Chuka Umuna - even though each of those MPs strongly disagree with Corbyn.
But there is a difference - those new centrist members of the mid 1990s actually got off their backsides, got out on the streets, knocked on doors and won general elections. No hope of the new Corbyn members doing that - far too busy playing armchair politics and having nice political debates across their dinner party tables. If they can't be bothered to turn up to a branch meeting, what hope is there of them pounding the streets and being abused on the doorstep while canvassing (as isn't uncommon). But then if you are comfortable to be perpetually in opposition so long as you retain your precious 'political purity' then you aren't actually interested in winning general elections.