I wouldn't compared GE20 to GE15 because that was an anomaly where voters voted for the sake of a dead man and shoved all bread and butter issues aside.
A fairer comparison is GE11 Vs GE20 because all issues at play are almost identical.
GE11 was historical low for pap but the impact of GE20 was greater for the fact GE20 was a crisis election but pap only fare marginally better than GE11.
If there is no crisis, pap vote share would likely dropped below 60%. Pap would likely need to get use to the new normal of sub 60% scores for future elections