As best as I understand it, no other sterilization techniques we have are nearly as effective as launching an object into space for several months or more. So, unless it's directed to portions of a spacecraft that are meaninfully protected from radiation, vacuum, and heat throughout the craft's voyage, pre-launch sterilization doesn't seem to have much purpose.
No sterilization technique is 100% perfect, and whatever contamination might happen is strongly determined by the one mission with the least effective sterilization, which is probably about equal as determined by the interplanetary environment, with variations for duration of flight and strength of solar radiation spikes (and assuming no significant variation over time in cosmic rays). To the extent pre-launch sterilization makes any difference, the earliest landers on other planets probably had the least effective sterilization techniques. For example, the Viking landers were sterilized by exposure to heat at 111 degrees Celsius, but the known upper limit of survival and proliferation temperature for thermophilic archaeans has since increased several times, with the most recent value I'm aware of at 122 degrees Celsius. The Soviet Union landed several probes on Mars and had a program of sterilizing them, but I'm not aware of the details. There was at least one science fiction story where astronauts on Mars discovered microbes that turned out to have evolved from microbes from one of the Soviet landers.
We also know that interplanetary space is not a sterile barrier over long time scales, and that there has been exchange of rocky material between Earth and Mars throughout the history of the Solar System, of which ALH84001 is a recent example. It's conceivable that radiation-hardened microbes such as deinococcus radiodurans could survive the radiation exposure of the trip, while the interiors of such rocks could protect them from the thermal spikes and mechanical shocks of excavation and landing.
Even what precautions our anti-contamination policies have included might have been counterproductive, such as sending the Galileo probe into the atmosphere of Jupiter to prevent the chance of it striking one of the Galilean moons in the future. Even if Europa, Ganymede and Callisto are all teeming with life in sub-surface oceans, it seems like having a 50-ish mile shield of ice between the ocean and a crash-landed probe sitting in the vacuum and hard radiation environment of space is pretty good insurance. But the choice was dictated by humanity's assumption of lots of liquid water as a paramount determinant of the possibility of life. That might be chauvinistic based on our biosphere sample size of one. If you relax your assumptions for the conditions necessary for life to arise and evolve to just a relatively stable space with chemical complexity and energy, the atmospheres of the gas giants become candidates. If you integrate the possibility of life first arising over a volume of space as well as duration of time for the candidate environment, Jupiter becomes overwhelmingly the most likely candidate environment in the solar system for life to have arisen. And unlike the thick ice envelopes of the moons, exposure of the Galileo probe to any portion of the atmosphere of Jupiter is potentially exposure to the entirety of the planet.
It makes sense to take reasonable precautions, but we can't expect to be able to ensure 100% lack of contamination from any spacecraft. And while there are good analytical reasons to believe there is no threat from back contamination to the Earth's biosphere from possible extraterrestrial microbes from a robotic or crewed sample return mission, there's no substitute for experimental evidence, in the form of living things from Earth living and growing on Mars without being hermetically sealed from the Martian environment. It would be fantastic to land at least a robotic greenhouse on Mars, like Chris McKay and then Elon Musk were promoting several years ago, and be able to watch plants and flowers from Earth growing on Mars.
Ultimately, it's not any more unlikely for Earth life to spread to Mars as it was for life on Earth to spread out of the oceans and onto land.