it’s not always difficult to remove. My old 2005 T&C van it was really easy to access right on top of everything. Each car is different. Bringing back 1960s technology? That’s a joke I hope. I’m going to continue to enjoy my modern safety equipment, seatbelts, power steering, electronic fuel injection, multivicosity oils that don’t require 3,000 mile and/or seasonal changes, cars that go beyond 100,000 and it’s not considered a miracle…(among many other things)
Not reading all the responses, a leak can be anywhere, not just the heater core. Even a new radiator could have developed a leak. Or could be hoses, water pump, etc. So the first thing is to find out where the leak is and repair it. Then after the fluid is topped off again and air purged out, your heater should work fine.