This could lead to a situation where most users will actually prefer an online-wallet that also handles their respective LN-channel(s), just to spare them the hassle of running something themselves.
Of course, that's the overall trend with any online software (e.g. email being used over gmail etc.), but LN kind of encourages that kind of delegation from the user's side to service providers.
On the other hand, users are free to run their own Bitcoin and LN nodes, just the same as with running their own SMTP-sever, so it's effectively more a kind of "concentration" rather than real "centralization".
Yeah, that is some good analogies you make there. Same as with exchanges, most users use coinbase and other online wallet/exchanges to store and use their bitcoin. How many actually download bitcoin core and run a full node? A very small percentage I think. On your phone you can't really run a full node either, has anyone a full bitcoin client with 200 GB of blocks stored on their android phone? no. But do people say bitcoin is centralized because of this? of course not.
Even with most users using a "light wallet", the network itself will be decentralized and yes, maybe we will have a thousand "big" nodes that have much more channels and much higher balance than the average users, but it will still be extremely decentralized.