SD cards have limited write cycles so it’s just a question of time when your pi will not start.
To increase their life, all manufactures have implemented an internal algorithm which uses different physical sectors when writing data to the SD-Card, see wear leveling.
You can help your hardware if you “oversize” the SD-Card, avoid ram discs and swap partitions.
I am using a 16GB SD-Card even though just 4GB is used by the system. It’s running now for more than 4 years without any trouble.