In particular, the smaller the GDP, the less military spending the country can support. If the country can not support a sufficient military spending owing to a lack of consumption (production), it will be conquered by someone who can, a bit like private equity buying out an unprofitable company with good potential. (c.f. Ukraine - very poor GDP, but decent possibilities for growth w.r.t. arable land, supply of young human capital, seaports, industry etc, thus currently undergoing hostile takeover)
> You assume people don't even consider saving somehow. Why is that?
There's no such thing as saving; if I save money, it's to consume it in the future eventually. So this has to cancel out over the long term.
If people are saving money and never withdrawing (e.g. never using the freedom bought by the past work to avoid future work), that's fine too, but if they withdraw it, it decreases national production, so it only works if people are prevented from withdrawing savings, at which point nobody would keep them.
Basically, people have to work, and this is the fundamental premise. That being said, the basic nature of the consumption is irrelevant. This is why digital services, gambling, and financial services are so important - it's much more socially efficient for people to dispose of $100 worth of money through slot machines or Netflix subscriptions or overdraft charges (all of which are effectively free to "produce" in terms of national resources) than by e.g. buying a new car, which requires actual steel and semiconductors and rubber and ... which require the expenditure of resources (land, labour, capital) in order to produce.