The miners are the ones securing the network and I think most of them can easily afford to buy more storage/bandwith.
This is wrong.
Miners AND nodes are both securing the network. Miners by providing hashrate and securing the network against 51% attacks and nodes by enforcing the consensus rules and keeping Bitcoin decentralized.
In this regard, less people running a full node isn't really a threat...
When speaking of number of nodes, block size, etc. saying things like "less" is ambiguous. The real question is "by how much?".
For example when discussing block size increase it should be clarified whether we are talking about a small increase (eg. 1 to 2 MB) or a bigger increase (eg. 1 to 100 MB), there is a big difference between these two. Same with number of nodes. If you could find historical data of nodes or a chart showing the count you could see that we have had big drops and slow rises in the past. For instance back when block started becoming full and again when there was a spam attack where mempool grow to huge sizes the number of nodes decline. But when the price started rising and bitcoin adoption grew the number of nodes went back up again.
So lets put "less people running a full node" into perspective. If now that we have 50k nodes it dropped to 20k, things will still be fine but if it dropped to 1k with only a dozen listening nodes then the network will be in some trouble that will only get worse with time.
Also the thing about block size is that it doesn't matter how much you increase it, in the end it won't be enough. So the trick is to increase it in a way that it minimize the
inevitable damage but not so much that it disrupts everything.