A profile with 60 reviews from three years ago and nothing since reads as less trustworthy - to both Google and homeowners - than a profile with 20 reviews, five of them from the last month. Review velocity (how steadily new reviews come in) is weighed differently than raw review count.
The practical implication: a one-time push to get reviews from every past client is worth doing, but it's not a substitute for an ongoing system that requests a review after every job. Automating that request the day a project wraps is the single highest-leverage habit for staying visibly active.