Top: cumulative ZEC balance in the Sapling shielded pool. Bottom: daily inflow and outflow lines. Sapling activated at block 419,200 and was the primary shielded pool until Orchard. The balance shows the net accumulation of ZEC in the pool over time.
The upper panel is how much ZEC sits in the Sapling pool; the lower is how much moved in and out each day. A balance is a stock and a flow is a rate. Reading one as the other is the mistake this chart most invites. A falling balance with healthy flow in both directions is turnover; a falling balance with no flow is abandonment. They look the same on the top panel alone.
Both sides are exact, not estimated. Value entering or leaving a shielded pool crosses the transparent turnstile and is counted by consensus, which is why these numbers can be stated precisely while nothing about the holders can be. What stays hidden is everything inside: who, how much each, and moving to whom.
Sapling's decline is mostly migration, not exit. It activated at block 419,200 and carried shielded Zcash until Orchard; value leaving it has largely gone to Orchard rather than to transparent addresses. Check the two charts together before reading a fall as people giving up on privacy.
For all pools on one axis see shielded pool balances, or net flows compared for the same question asked of direction alone.