My TFSA contributions grow, without my having to pay taxes on the growth they experience.
Is this benefit likely to continue indefinitely, or will I at some point be required to pay these taxes?
After reading the comments in your question you should ask under what scenarios is there likely to be a change. Keep in mind that people have about a trillion $ of unused RRSP room. For the vast majority of Canadians that would be a better investment anyway.
The most likely thing I see is that the OAS/GIS system will change if people start accumulating large amounts in the TFSA and retiring early to use up the RRSPs before age 67 or 71. At that point their TFSA withdrawals won't affect the amount of OAS/GIS received. This could mean essentially paying welfare to millionaires. Look for a means test on social security programs in a decade or so.