It may have looked easy, and the act of putting a cassette into a player is very easy but a lot of work and constraints go into having IFE on an aircraft.
Cassettes are a dying media format that are now very costly to maintain and repair, especially on aircraft. A lot of airlines used Hi-8 tapes which went out with the dinosaurs.
The time has come where these machines needed to be changed to DVD players-DVDs are not has hard wearing in an aircraft environment with multiple users as they are at home with their owners, that added to the actual cost of purchasing, getting them certified and fitted to different aircraft, that may be leaving the fleet very soon in relative terms becomes a costly exercise.
Many of the scheduled airlines customer profile doesn't lend itself to IFE users on overhead monitors-businessmen who don't want to be disturbed or people who just want to get from a-b.
in regards to the charter operators many of these sell headphones which have to either subsidise or pay for the systems, many people are wise to this and bring or buy their own from outside the airline therefore it doesn't become cost effective to invest in new systems for charter operators or scheduled when so little customers value them or are willing to pay for them. As licenses are expensive, one reason Thomson airways when they use 767's with seat back IFE on all but long haul routes they are turned off as Thomson doesn't pay on mid/short haul routes for the license.
Hope this helps,
Tv monitors are not always present to show safety messages, manual demos are sometimes carried out.