Mechanical failures do happen with aircraft. Even the Poles conducted a comprehensive investigation on the aftermath of the plane crash, and they didn't found any evidence to support the fact that the crash was caused by Russia. And a lot of presidents have died in plane crashes in the past. Examples are John Garang (South Sudan), Joseph Kabila (DRC), and Juvénal Habyarimana (Rwanda).
I've read about it and it's not completely clear. They haven't found any proof of Russian involvement, but there are some unexplained things in that crash.
1. The prime minister of Poland met with Putin prior to the crash and the two had a private conversation. Later on the Minister and some of his closest advisors, including the person responsible for preparing and securing the whole journey didn't board the presidential flight.
2. All cameras and detectors on the Warsaw airport were disabled on that date, there are no videos from the day before, when the plane was being prepared and fueled. There are no lists of people who accessed the plane. Where was the security?
3. Parts of the plane were being found far from the crash, which means some of them had to break off before the crash. How did that happen?
4. Russia declined to give back the wreckage and had it scrapped, even though Poland wanted it back and wanted to cover the costs of transportation.