There are a lot of users who use the Bitcoin core wallet.

For a fully verified node，they face a huge problem, that is, more and more hard disk space. up to now（2018）, more than 200G space is required. cpu and memory，it is continuous consumption and will recover after use，however, the hard disk space is a hard requirement and cannot be released after being occupied.

The beginning of Bitcoin, 2009--2012，the hard disk space less than 1G,
in 2015, 20G space is required for a full node.
now, more than 200G.
so, follow this, more and more nodes can only be simple payment verification nodes. Machines running full nodes are concentrated in the hands of large disk users, like a mining pool and ASIC, contrary to decentralization, will also become a security risk for Bitcoin.