The US would be completely destroyed by splitting into 50 countries in any practical sense. Just look at the US under the Articles of Confederation.
The US would go from having the most powerful military on earth to a complete inability to fight anybody without a central government. Hopefully Canada wouldn't invade, but southern border states would have much less funding to stop illegal immigration.
Without a military, central government, or giant economy, the US's soft power would be gone as there's no longer a singular foreign policy or reason for other countries to really care. Along the same lines, the states would all be pitted against each other in any trade deal negotiations. There would no longer be any tariffs or regulatory motivation to buy something from America over buying it from Canada or China.
The Interstate System and rail systems would be greatly impeded. There would be no more consistent driver's licenses, speed limits, car safety or emissions standards, seat belt laws, or DUI laws. Just imagine trying to transport vegetables from California to New York as you now have to go through 10 international border crossings, all with totally different laws.
Medicare, social security, the ACA, the VA, FHA, CDC, NASA, NIH, and USDA would all be gone. A lot of people get help from those policies and agencies which they no longer would. In addition, without federal funding, food scarcity would become a real possibility. Currently the federal government spends a lot on subsidizing food production to ensure a stable food supply. Without that, the Midwest and Great Plains would first of all be a lot poorer, but also produce much less food. So food would be more expensive for everyone and a bad drought could legitimately cause starvation.
Trade would also now involve 50 separate currencies with 50 separate organizations managing their monetary policies, all without FDIC insurance, SBA loans, or Secret Service counterfeiting prosecution.
Without any federal Constitution, FBI, FHA, Title IX, Brown v. Board of Education, or RFRA, there would be much fewer checks and balances to prevent injustice. If your state legislature passes a law or removes a protection from its Constitution, that would be it. There would be no more First Amendment or Fourth Amendment or Fourteenth Amendment to cite or SCOTUS to appeal to.
Some of these things could be replaced on the state level, but there's no guarantee they would. Plus that would then have at least the same costs as we have now but with a bunch of redundant departments for every state instead of just one overall department.
Obviously the federal government has problems and I'd like it to be smaller, but that would be cure far worse than the disease. When the Confederacy tried to secede so they could keep slavery, Lincoln fought the Civil war not to destroy slavery, but to preserve the union because he saw that as the "paramount object". (Although personally, I think abolishing slavery is more important than any success as a country.)