some thoughts from a higher timeframe trader:

Do not confuse backtesting over long periods to establish positive expectancy and optimisation to tweak a little more profitability by keeping your parameters in tune with current conditions.

What do we mean when we say markets change over time? Markets trend, range or chop so are we saying that the % of time they spend in these states changes? I think that is what we mean, so we want to know how well the strategy copes not only when the mix suits our approach but more importantly how badly it does when the market mix doesn't suit. I'm leaning for more history.

A completely different problem we have is defining our market mix, what mix of trend, range and chop gave periods of high profitability or low profitability. If we define this, we can look into mechanisms to switch on or off the bot appropriately, be they filters or equity curve trading. Only then would I look into optimising parameters, and I would be wary of where the breaks in my WFO cycles fall. 2008 is the type of year where I would want the bot switched on or off not contributing sample data for optimising parameters in 2011 for example. I'm still for more history, but cautious about optimisation.

Finally, beware permanent changes over time when backtesting. A 0.1 change in the S&P in 2007 is a more significant percentage change than 0.1 in 2013, this can affect stops, targets, certain indicators, and equity. So I'm still with more history, but algos need to specifically allow for permanent changes.

Edit: was composing this when acid posted, apologies for making similar points. Also I wouldn't expect my comments to be valid on shorter time frames, but I have little experience of this.