You have to have unfiltered BGP access, which any sane transit provider will only provide to other major transit providers
........ Sorry, I need a few more seconds to compose myself, stop rolling on the floor laughing, and get back on my chair before I can reply. ;)
1. You're assuming that the internet has clear borders between "anyone with a BGP router" and "major transit providers". You're assuming it is difficult for anyone in the out-set to persuade anyone in the in-set to let them in. Bear in mind some parts of the "transit provider" set can be large clusters of quite small players (sometimes literally 1-man operations). Bear in mind the internet has been growing at a good pace, and is likely to continue to grow for some time, and that many in the "transit provider" set have a business model that depends on that growth happening.
2. Ignoring point 1, taking it as given a clear delineation criteria between the transit ASes and the edges (clear from the POV of the transit providers), and a transit provider set which are *all* strongly motivated to exclude any new entrants: You're assuming that a large percentage of transit providers are both, a) technically competent at specifying filters b) have a clear financial motivation to spend their resources on implementing operational processes to ensure new non-transit customers will consistently have filters applied. Even if the vast majority of transit providers meet assumption a (and it's not clear that's the case ☺), assumption b doesn't hold for most (at least, it's not immediately obvious to them). So, there isn't really a great pressure on them to reliably implement these filters, and so often they don't (because they generally don't, or because their processes aren't rigorous enough to ensure they reliably do).
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