OK I accept that I'm on the extreme end of the scale here having only slings and no buggy, but there are a load of myths flying around here!
With a healthy child and a healthy mother, there is very rarely such a thing as too heavy, and when it occurs the child is usually old enough to walk the majority of/all the time. It's natural progression, as the child gets bigger, they are carried less frequently and for shorter periods. Whenever they want to be on their feet they can be, and you shove the sling in your bag, and vice versa. My 4 year old is down to one or two carries per week. It does not knacker your back/hips/insides if you are otherwise healthy. I have found the exact opposite - that it strengthens. There should never be pain or strain - and if there is, and you want to use your sling/carry, you should seek help, leading to my next point:
Speaking as someone who runs three sling meets. Baby carrying is something of an art form, espcially wrapping which can be incredibly simple, or like baby origami
Carrying a 4 week old is different to carrying a 4 month old to carrying a 4 year old. It evolves. Most slings and carriers will evolve with your needs but you may need help in getting the most from them, being safe with your carrying, learning new ways to use them. The average sling meet attendee is there for information - pregnant and looking to try before they buy, new to slings completely and looking to find something new, used to the sling they have but wanting something different, or wanting instruction on how to use the sling they have, either from scratch or in a different way, for example on the back.
I'll most often see someone to show them good supportive carries with a young baby, who will come back a few months to learn back carries with a bigger baby, who will come back a few months on to learn short up-down carries for their new walker.
There are a few people who come regularly (obviously this includes me) for whom the meet is not about slings any more but a chance to see friends and eat cake. As a group there is more knowledge (and more slings to try) to offer to those who come occasionally. It is not a cult, nor lentil weavery, and the people who come are from all walks of life, and of all appearances, with all parenting methods imaginable. Just like any other group.
I do not have a car, and with regards to the shopping, I've never had trouble bringing it home pushchair-free. With regards to the local shopping point, there is no way I could have taken a pushchair in and out of our tiny, maze like local shops in a big steep hill. In the butchers and the grocers most people leave buggies outside on the main road (often with babies in) while they nip in to get their things.
My home is so small that if I brought a sleeping baby home in a pushchair I would not be able to bring them in to the house!
And nowhere to store even a folded pushchair.
Most people will use a combination of the two, which is great. But please don't dismiss slings and/or the sling meets out of hand - they are not a mad hippy invention/convention but something designed to help and be very useful 