NATO has a few purposes these days, its original purpose was primarily to link Western Europe with a promise of an immediate American military response to a Soviet (Russian) incursion, and while not to denigrate the other members of the alliance, that is still the lion’s share of the NATO deterrent. Part of that is because having an external superpower as a protector is one thing that all of these countries essentially agree on that they want. Without the United States in the alliance, it is likely it would fracture apart significantly (and I doubt Turkey would be in it, for example–most of the EU would, though.)
In that regard, a bilateral security arrangement between the United States & Sweden and the United States & Finland, does indeed satisfy much of what is desired in terms of countering current Russian aggressions. In fact this isn’t materially unheard of–the United States has bilateral security agreements with two significant non-NATO allies–Japan and South Korea.
We have a 1960 Mutual Defense treaty with Japan–an attack on Japan is an attack on the United States, functionally.
We similarly have a 1953 Mutual Defense treaty with South Korea.
Part of these treaties, much like NATO, is we do regular exercises with the relative military forces of these countries (more limited with Japan as they have a “Self Defense Force”, and constitutionally reject any form of non-defensive war); among other things to keep these military associations tight.
We also have a “Collective Security Agreement” called ANZUS, which is technically between the United States and Australia, and Australia and New Zealand, but it is not a mutual defense treaty. It is highly likely if Australia was ever attacked, the United States would defend it, and we have a lot of close military relationships and activities, but it isn’t technically a mutual defense pact as it is not binding on either party. (The treaty used to be a three way pact, but the U.S. severed its specific portion of the pact with NZ in 1984.)
The U.S. could certainly do this with Sweden/Finland, and likely arrange for military standardization and etc between the three countries to provide similar to NATO benefits.
It would not have all of the benefits of NATO, but the core mutual defense agreement with the United States would be there.