In the absence of the Russians, Iran has been forced to spend on defending Syria.
Russia has long transferred a significant portion of its missile defense systems from Syria to Ukraine to defend against attacks by Ukrainian defenders during its aggression against that country. This was at a time when Syria and its army mostly relied on these systems to create a deterrent against Israeli attacks, and in many cases, the presence of systems like the S300 in Syria merely served a deterrent role, as the Syrians were unable to use them.
Reports indicate that Iran has transferred air defense systems to Syria. Part of what has been transferred are systems known as Bavar 373, and another part consists of systems that Iran has purchased from other countries, specifically China. Last month, Syria successfully used some of these systems.
Israeli attacks have intensified and achieved unprecedented success in recent months, and apparently, several Iranian officers have been among the casualties of Israeli airstrikes. Russia has even imposed the secondary costs of its aggression against Ukraine on Iran, forcing Iran to spend on defending Syria, where it has practically lost much of its influence.