(am brit)
Is this not already true? When my laptop was stolen, I was advised "don't bother reporting it to the police".
Given the expense of rolling out the new coin, it probably makes more sense from a economic and moral point of view to spend money investigating these crimes rather than preventing them.
We also don't know anything about the volume of forged coins withdrawn from the market each month. If detection at pubs, grocery stores, etc. is poor, then fake coins enter the market at will and are detected only when merchants present their cash to banks. That's annoying for everyone, and it's possible to come up with scenarios in which each year sees 2x or even 5x the introduction of fake coins, with many of them staying in circulation for only a few months but causing damages anyway.
In either of these scenarios, clamping down now on forgery is economically prudent, even if the 100 > 45 comparison doesn't reveal the true cost.
But I would guess from a public relation perspective, asking a whole population to replace their coins just to make it cheaper and easier for banks is a harder sell that arguing security.
The cost of changing comes primarily from businesses having to update vending machines, coin counting machines, etc (as well as increased production costs due to the new security features.)
But this switch has been planned for many years, so businesses have had a long time to prepare.
By comparison, the new, smaller coins are much harder to grasp and the new denomination (2kr) is hard to tell apart from the new (1kr). The new notes are similarly hard to tell apart, as they all i) have the same height, and ii) use similar (pastel) colors.
"Never attribute to malice what you can blame on stupidity", but it almost feels like they are making the new cash hard to use on purpose.
Meanwhile the entire currency is debased annually by failing to include land prices into the inflation calculations, despite most credit creation being against land.
This debases GBP for workers every year. Therefore propping up confidence is vital.