There is no KBG now of course, but the guy who used to be a part of the old KGB is dictator and chief of Russia now. Do you think he would abandon basic aspects of what has always made spying a workable proposition?
What is interesting here is how easy it can be starting out; especially if you have someone who is either compulsive, prone to risky behavior, always in need of favors, or some combination of all of the above.
Say, if you gambled a lot, had significant debts because of that, and were always in need of cash to make ends meet, you might find yourself introduced to a party with money, but ostensibly representing some group other than a foreign power. They get you to a meet to talk about how they can help and, as innocent as can be, just happen to push an obviously full envelope over to you. Something you might have been given to believe is simply a proposal. As you take that envelope, however, and put it into your pocket, pictures are taken.
Later on then, when what they want becomes a great deal more brazen in illegality, and/or treasonable acts, and you then try to balk at compliance, they present the pictures to you, telling you something you didn't know at the time; say that one of the people you met was a known Russian spy by Western intelligence agencies. At which point, whether there was money in the envelope you received, or some other not very innocent material, hardly matters because the intelligence people in your own government, let alone people in the media, or political opponents, would find it very suspicious regardless. And just the mere hint of such suspicion, with visual evidence, would end your career no matter what. At which point they have you in their pocket, and the next steps of actual illegal acts on your part become inevitable. Partly because you feel you can't go back now, and partly because they assure you that you won't get caught if you do things exactly as they tell you to do them.
I mention this because it provides reference to how laughable the current denials from Russia that "the Kremlin doesn't collect compromising information," as Dmitry Peskov tried to exclaim in response now to growing reports that they have something on Putin's favorite new ass kisser. You know, the guy about to take up residence in the White House? Mere semantics in the cause of plausible deniability as what passes for the KGB now certainly does collect such information.
The bottom line here is simple. All of the allegations have legs now for four main reasons:
1: Putin and the Russians have been the recipients of beneficial behavior from trump
2: Putin and the Russians have been linked very specifically to screwing with our presidential election; machinations that may very well have helped Trump win (not to discount Clinton's own responsibility for running a poor campaign, which made the contest closer than it should have been).
3: Trump refuses to come absolutely clean on all of his finances, including the release of all tax records, as well as details of all foreign investments.
4. Trump himself just can't seem to let the issue go, doubling down time and again on not only contradictory denials, but on keeping up with the praise for Putin and the Russian government.
As such, the degree to which this issue still has legs is nobody's fault but Trump's own.