Both Lego and Playmobil could do a better job with their spare parts ordering system - I've tried ordering spare parts using Lego's online system and it's still not ideal with a very limited range available. At least with Playmobil if they have it, they's sell it to you, but it's hard to find if they have it - made much easier by Heather's database.
The question is, is it worth it to these companies to invest more effort and time in improving this area of their business (spare part & replacement part sales), and both seem to have answered "not at present".
It's a customer service issue, and only affects a tiny proportion of their customers who actually bother to try to order spare parts.