The deflation due to the reduction of supply (permanently lost bitcoins) will be slow. I doubt that more than 1% of the entire world's wealth will be lost in bitcoin transaction mistakes. Stocks, bonds, and other investments generally have a much higher rate of return than that. When a business is using bitcoin to purchase stuff, and accepting bitcoin for the sale of stuff, and holds bitcoin on their balance sheet, then any increase in the value of bitcoin (due to deflation) will increase the value of the company IN ADDITION TO any profit the business generates from the products or services it provides. Therefore, investing in a business will be a way to increase the amount of bitcoin that you hold.
As such, I expect that funds beyond those which an individual needs immediate access to will still be invested into growing the economy (and one's personal net worth).
Remember that, right now, people still think about value in terms of their local currency. The fact that 1 Bitcoin used to be the value of a Big Mac and is now the value of a new car isn't how people think of it, they think of it in terms of how many dollars (or euros, or yen, or rupees, etc) the Bitcoin is worth. When Bitcoin is ubiquitous, people won't think as much about how many dollars their bitcoin can get them (since it won't matter). Especially if that value (relative to the things it can purchase) is growing at a rate of 1% or less per year. Instead, they'll think about their net worth in terms of Bitcoin itself. Growing one's net worth will mean growing the amount of Bitcoin one holds. That can be accomplished by working a job that pays bitcoin, and spending less bitcoin than you earn. But it can also be accomplished by investing some of that unspent bitcoin in a business and sharing in its success.