Not sure that's a fair characterisation! Under "Big Jerry's" watch as DefMin 2014-2017 he:
Advocated for the C-17 purchase years ahead of the formal air mobility replacement project timeframes (due to the C-17 "whitetail situation") and even managed to swing a
RAAF C-17 visit to showcase the capabilities to his fellow MP's. Interestingly Labour supported the C-17 purchase but ironically the "pro-defence" NZ First party didn't. (Not his fault the bean-counters at Treasury were non-supportive of the NZDF's plans to fund the purchase (eg by reducing the 757 fleet) and kept insisting they look for other savings, which stalled Cabinet approval until it was too late when 2 of the 3 remaining whitetails were then sold to another buyer)!
But in terms of successes he managed to have defence spending increased in 2016 and got Cabinet sign-off on the original
$20b CapEx investment plan also in 2016, Cabinet approval for the Army's Modular Assault Rifle System - Light (LMT CQB16) to replace the Steyr's, new SAS vehicles (Supacat Extenda high mobility vehicle ) the Navy's AOR tanker replacement, the Navy's Littoral Operations Support Capability (not his fault the next Govt bought a cheaper second-hand vessel!), $1.7b for defence
estate regeneration, finally the decades long delayed upgrades for the P-3K Orions ASW/underwater ISR systems and supported the NZDF training mission in Iraq etc.
He actually got a lot done for the time (which was when the Govt was prioritising achieving its first budget surplus in years and were pretty "stingy" in many other portfolios)!
No reason why van de Molen couldn't get the defence portfolio back if there is a change of Govt, but the postive news is there are plenty of other excellent candidates (former NZDF/ADF officer Chris Penk would be an excellent choice but is probably earmarked for other senior roles). But frankly I would give the role to ACT if I could because that way the NZDF would be supported very, very well (kind of like how NZFirst supported them rather well in coalition with Labour 2017-2020) ... well assuming that's a coalition condition of course.