To be even clearer, maintenance is the operator’s responsibility, not the manufacturers. Once the bird leaves Airbus, Boeing, Bombardier, Embraer, whatever, the customer takes on the job of keeping it airworthy.
That's not entirely true.
"Responsibility for Airworthiness. Title 14 CFR part 91 states that the owner/operator of a civil aircraft is primarily responsible for maintaining that aircraft in an airworthy condition" [emphasis added]
You'll notice, it doesn't say things like exclusively responsible, or only responsible. The carriers are primarily responsible...but they are not the only ones responsible.
What "inspection program" are the carriers using?
Their own (the carriers)? Meaning, the aircraft in question is no longer "supported" by the manufacturer? Examples - ex-military aircraft, low-usage aircraft, or special operations aircraft, such as Forest Service smokejumpers or firefighting.
Or, is the aircraft still being supported by the manufacturer and therefore the carrier is using the manufacturer's "inspection program?" Is that manufacturer's inspection program lacking in any way? Has the manufacturer lessened or decreased any part of their inspection program?
What about parts? If an aircraft needs to have parts replaced...who manufactured those parts? The carriers? Unlikely. The aircraft manufacturer? Are they OEM? Are those replacement parts made to the same standard as originally designed? Even if a carrier properly installs the new part(s), per manufacturer specs, if the new parts are sub-standard, the likelihood of failure increases.
Was an inspection carried out by the manufacture itself (or one of it's subsidiaries)? Did the manufacturer rebuild any part that was subsequently re-installed? 14 CFR § 43.3 - Persons authorized to perform maintenance, preventive maintenance, rebuilding, and alterations.
It's far more complex than simply...Boeing made a plane and once the carrier purchases it, Boeing has zero responsibility for the continued airworthiness of the plane.
There's a common theme amongst what seems like a significant increase in "issues" with aircraft these days...and that common theme just so happens to be Boeing.