Spam will always be an issue. No matter what limit will be set, block creators will always reach it. Set 1 MB as Satoshi did, and it will be reached. Set 4 MB as Segwit creators did, and you will also see fully filled blocks.

If you are a block creator, then you have an incentive to create the biggest possible block. Why? Because then, you can pick some simple, deterministic algorithm, and generate terabytes of always valid transactions on-the-fly, then let your miners work on that header, and send mined block to other nodes. Then, you can start producing some next block, on top of what you created, while other block creators will try to validate what you submitted.

Instead of thinking about block size alone, think about verification time. If blocks are produced every 10 minutes, but your blocks are so complex, that it takes 5 minutes to verify them, then guess what: other block creators have 5 minutes of "guessing time": you already know, if your block is valid or not. But others don't, and they have to decide, if they want to create the next block on top of not-yet-checked-block.