In addition to what MDM quoted, keep in mind that you can always withdraw your Roth contributions at any time with no tax or penalty. So if you're worried about access to your funds, you really only need to worry about any investment gains between now and the time you would want to withdraw. Roth gains, if withdrawn early, are subject to tax at your current marginal rate plus a 10% penalty.
For this reason you really don't want to get yourself in a position where you have no taxable funds, you've rolled over all of your pre-tax retirement accounts to Roth, you've withdrawn all of your Roth principal, and all the money you have left to your name is Roth gains. If you think there's a danger of getting into this situation, don't contribute to a Roth account. However I would posit that for most of us, if we have gotten ourselves into this situation prior to age 59½, we have bigger problems (aka we're about to run out of money completely). I view my Roth gains as "old age money" that will be there for me to use after "normal" retirement age, and most likely won't be needed at all (since the studies that led to the 4% SWR show that you have good odds of not even coming close to running out of money during retirement if you only withdraw 4%).