The country of 28m has to pay for the existing infrastructure. The older the infrastructure, the more maintenance, and eventually replacement (in segments) is required, And of course, as the population expands, the network has to expand with it. I would love to see the transmission and distribution maintenance and upgrade budget over the years to compare incremental upgrading/installing upgrades to handle renewables over a similar period Rome, after all, wasn't built in a day, The stats provided by @octave already bear out the major cost of your bill is infrastructure, so it would be interesting to compare that cost to a new build and its projected costs over time.
You also speak of this under-utilisation of capacity - which is not quire accurate. I am sure there are times the grid is underutilised - for example, around 3am Easternm - this would be factored into the price you pay. All utilities are underutilised at some stage. Yet it is even more expensive with coal, as you have to keep those furnaces burning.. That is under utilisation. I think what you mean and I may have misread it - the cost of writing off the capital before the end of its useful life. Yes, that is a cost, however it is borne from continual investment in obsolete technology. And isn't the grid being upgraded for renewables, and transitioning rather than abruptly stopping legacy network infrastructure? Sounds like they are trying to make the transition (as oppose to switching) from legacy to upgraded grid as cost efficient as possible. But this sort of thing happens anyway, as even with legacy infrastructure, components time expire, become obsolete and are replaced (sometimes before their useful life if the benefits of replacement technology can provide a quicker economic return).
Then there's the extensions to nuclear plants. This is not a simple visit from the NRC or NII (as it was called then), a few patch jobs and biob's your uncle. I was involved in a two life extensions (not the whole thing). They are years in the planning and delivery and are major refubrishment programs. Both cost well above USD$600m and that was (jeepers!) 25 years ago. Typically, plants have two generation facilities - and that well above $600m was for each facilitiy (which is why they do one at a time). So, yeah, you can get life extensions, but they don't come cheap and are still full of risks to budget, timelines, etc.
Thee was mention of it's great if one can afford the subsidies for renewables.. I am not even sure what the issue is here. Virtually all new nuclear builds, at least in the Western word are subsidised or guaranteed one way or another. The LNP caolitiion's plan was to significantly subsidise the new builds in Australia. Great if it can be afforded. Hinkley Point and Sizewell C - Government guaranteed and guarateed price post commissioning, indexed, which are eye watering. Have a look at tax credits, government grants, loan guarantees, retail bill levies, etc that all prop up the industry in these countries.. As you say, great if you can afford it.. Obviously it can be afforded.
Lastly, no other country has relied on intermittent generation? No one had done much more than jump of a tree or a cliff,yet now, through technology, people fly safely. What sort of argument is that? It hasn't been done before, let's not do it? Is that really your argument.
Everything else @octave has dealt with competently.
Keep investing in coal - lets see where you are in 20 years time.