At least within Europe, we are on the era of advanced purchase discounts, yield management and online connectivity everywhere. Those who decide to travel as if it were 1989 suffer the financial consequences on transportation and accommodation costs. More and more high-speed trains charge really hefty fees for last-minute departures.
The omniscience and prevalence of Booking.com, which has very extensive coverage in Europe from global chains to B&Bs and small pensions, changed hotel pricing a lot. Even small hotels now can, through a couple clicks, engage in dynamic pricing, provide discounts to advanced non-refundable non-changeable reservations, and set the boundaries of prices so an algorithm takes place of repricing them thrice a day to follow competition.
I follow, just for fun, the pricing of hotels in Amsterdam, it is amazing how they change often, more than airfares. It is not only individual hotels changing prices, but dozens of hotels that let Booking.com manage their inventory as the primary interface, so you see wild fluctuations regarding discounts.
This change on accommodation pricing heavily impacted the feasibility of winging it on the less expensive strata of hotels. If you can spend € 400-500/couple/night (Northern/Central Europe/Italy), you will probably find somewhere reasonable on every major European destination except on major event dates. If you want to keep it below € 250, then it becomes trickier: what is to be found at last moment is usually of poor price/quality ratio - and some 'bottom feeder' operations specialize on providing overpriced beds on major cities for last-minute travelers.
In any case, this is my philosophy: I like to travel with accommodation and transportation set. I prefer to reduce the number of "changes of bed" and do more day-trips. That way, I'm more flexible within each stay to adjust for weather, spur-of-the-moment decisions, and so forth.
Then, I research my destinations a lot before traveling, reading what is available, when things are opened or not, and go on TripAdvisor and else to decide what is prudent to reserve in advance or not. I like to have a long list of possible things to do in or around places I'm staying, so I can decide on site.
A word of caution, though: a rapidly increasing number of busy venues (world-class museums, attractions with crowding problems etc) are moving towards time-slot advanced reservations. These things get rid of long waiting lines, and in some cases, deliberately restrict the overall number of visitors to a lower threshold than before (e.g., Anne Frank House in Amsterdam).