If you keep Bitcoin 1MB block limit, people will be forced to use other altcoins as well, so you basically have to download all blockchains anyway because not everyone is going to accept Bitcoin, so you save nothing from this point.
In your logic you could download only what you're interested in. Not *all* chains.
Which is actually better than having to download one massive bloatchain.
I know, but if someone need full blockchain for Bitcoin it is reasonable to expect he will want full blockchain for whatever coin he may be required to use. I can argue having one bloatchain is much easier than manage tens or maybe hundreds different blockchains. It is the same as if there is one fiat currency accepted everywhere worldwide versus current situation with hundreds of fiat currencies worldwide. This is why I preffer just one coin, one blockchain. As long as you can manage the blockchain on one average or better PC, it is no brainer choice. You can run current Bitcoin full node even on very low end PC, 1GHz CPU, 1GB RAM, 100GB HDD...