Twenty years of peace is much better than no peace at all. Twenty years is enough time for a generation of young men to forgo military service, time to build infrastructure, time to consolidate power, and time grow a treasury.

There are also plenty of examples of peaces that last longer than twenty years, or even result in permanent peace and consolidation. The nation of Spain was formed from the union of the Kingdom of Castile and the Kingdom of Aragon, a union that was set in motion when Isabella of Castile, the future queen, married Ferdinand the Catholic, a future king of Aragon. Their grandson Charles V and great grandson Phillip II would later become kings of a united Spain. James the VI of Scotland similarly oversaw the personal union of England and Scotland when he inherited the crown of England, becoming James I, in 1603. England and Scotland would later formally join together to become the Kingdom of Great Britain in 1707.

Your question also implies that it is particularly unusual for a peace treaty to last less than twenty years. There are numerous examples of more modern treaties that did not maintain peace for much longer than twenty years. There were only twenty-one years between the World Wars (1918 to 1939) and only twelve years between the Gulf War (ended 1991) and the Iraq War (began 2003).

I can try to include better sources if asked, but I don't think anything I've said here is controversial.