It's better than nothing, even if partly because other parts could - maybe - be built later
I think honestly any new bit of infrastructure we need to grab with both hands, even if only to keep it there for a benefit further down the line.
The more of the project that is lost, the harder it will be to recover. And the further into the race to the bottom we end up; a poor economy with barely any investment, hamstrung by poor public transport and the political consensus on actually building transport infrastructure now ripped up. It could take years to recover from this decision. The more that is still built, the more chance a future government will resume the whole thing.
The government right now needs no further encouragement to bin off anything - it has been their goal for a long time.
If they wanted to build this, even the raised cost - and everything has increased in cost due to inflation - isn't at all insurmountable when talking about national-scale funding and considering the benefits that we'd get afterwards.
Off topic for OOC I suppose, though what a stupid* decision to cancel any part of this, quite honestly. The same attitude that says we "can't afford" to stop schools falling down says we can't build a new railway line. Public financing isn't a household budget, they know this, so cancelling it is a political preference not a cost issue.
*Other less kind descriptions are available