Question I have been a bit scared to ask because people here jump down your throat if you dare be different.
If lightening introduces cheaper/free-like transactions? wont that diminish the value/volume of expensive(price driving) bitcoin transactions on the main chain?
I have thought enough about this to have some things to say. I'll add more later if I can think of more.
The bitcoin blockchain is an immutable eternal database. You can store something in there and be confident that that information will be stored forever. That is crazy useful. Property titles, ownership tokens, other blockchains will offer this of course, but bitcoin will always be the most ideal place.
Also we can always just not increase the block size, ever, until we feel like miners are more than fairly compensated through transaction fees and all models predict that a modest increase in block size will still reatain adequate network security. In fact we can base our decisions about when to increase the block size on no consideration other than security. I personally feel that everything else is secondary.
Also I don't think there is a world where lightning gets to the point where it isn't unjustifiably cumbersome for large transactions. Nation states that want to use bitcoin as an alternative to SWIFT will probably find the blockchain it's self to be the place to do something like that. So there will still always be a marginal demand for on chain transactions.