Tyrell Williams is the quintessential sleeper free agent. He is days into his 27th year and therefore can be construed as being in his prime. Should they chose not to re-sign him, the Chargers will have reasons for not re-signing that do not indicate lack of performance by Williams. Keenan Allen was elected to the Pro Bowl in 2017 and 2018, and is under contract through 2020. Mike Williams was the seventh overall pick in 2017, performed roughly equally to Tyrell, and is under contract through 2020.
It could said Mike surpassed or supplanted Tyrell down the stretch, but overall their respective performances are very very similar.