Fossil fuels will get more costly for straightforward reasons of supply and demand.
Unfortunately it is not quite this simple. The current oil price is on the order of $100 per barrel, but it never broke $40 per barrel prior to 1998. See figure. Also see this figure which is in terms of inflation-adjusted dollars, and shows another huge spike around 1980. The reason for these tremendous spike in price isn't simple supply-demand - complex nonlinear political factors are almost certainly to blame, and price stickiness is partially why oil remains as expensive as it is. It doesn't cost even in the ballpark of $100 per barrel to get oil out of the ground and it won't for a very very long time. The upshot is that the price of oil will continue to beat out other sources of energy by just enough to keep those sources of energy at a marginal level of profitability, because oil (and other fossil fuels) can remain profitable at much lower prices.
I would also point out that the scenario you have just described is highly complex and conjunctive, while "oil continues to do what it has been doing" is an intrinsically simple hypothesis.
Price is set on the margins. The marginal barrels of oil coming out of the ground are certainly in the ballpark of $100, from various shale and tight deposits.
As per a recent comment this thread is meant to voice contrarian opinions, that is anything this community tends not to agree with. Thus I ask you to post your contrarian views and upvote anything you do not agree with based on personal beliefs. Spam and trolling still needs to be downvoted.