If you are careful you can maintain the starter culture for a long time. Using glass jars makes it relatively easy. You'll need to have 2 jars (at least). Boil the jars (with lids) along with a metal table spoon and let them cool. For a thermophilic culture (high temp), wait until the milk cools down to 50 C (122 F). Open a new container of yogurt and use your table spoon to mix a spoon of yogurt into the milk. Pour the milk into the jars. Seal the jars. Always make sure to have one untouched jar available for seeding the next generation. Always seed with yogurt from a just opened jar -- don't let the jar sit around open.

After the milk in in the jars, stick them in a picnic cooler. Pour in a water bath of 50 C (122 F)water about half way up the sides of the jars. Close the lid and wait 8 hours or so. If you want, you can replace the water after 4 hours, but after fiddling with things I've found that I don't really need to do this. YMMV.

I've kept yogurt cultures this way for years and years with no noticeable drifting. Yogurt sealed this way will keep in the fridge for a surprisingly long time too. While I don't recommend it, I've tasted it after storing in the fridge for more than a year and it was fine. You can do something similar with mesophilic (room temperature) cultures, but you have to be more careful because there are more bacteria around that are happy at 25 C than 50 C.

i used to just take the left over yoghurt from the current batch to make the next one. the result wasn't good.

then i switched to freezing a few spoons full from a fresh batch. any small containers will do.

i got to 25 generations at one point. and the frozen yoghurt kept fresh for prolonged periods of time, like when we went away to travel...

You are correct. This is also a prevalent myth in the bread making world with sourdough starters.

i think it's less of a myth but rather that knowing how to get yeast from scratch is a rather recent discovery, and before that keeping a starter from one generation to the next was probably the only way to make sure you always had some around

Today you can buy yeast anywhere. But many hobbyist bakers take pride in their yeast and keep and maintain them in their fridge like pets. It may not be entirely logical, but I do understand the attraction, it's like having your own little buddy hanging around that helps you cook, personalizing all of your baked goods.

exactly. i think once you are in the process of making it, keeping a starter around is also more convenient than buying a fresh one every time.

it's like why even bother going to the store...

the point was that a myth is something that hasn't been true for ages if it was true ever.

but keeping starters alive was certainly true for a very long time until very recently.

Each chain is different, I don’t think they all evolve the same way. And I think the flavours do settle, or rather each family’s heirloom is one evolutionary pathway.

There’s other examples in cheese (the flavour of a small batch maker) or beer as well, I think.