I don’t think anyone is saying the universe is infinite, but that it might be expanding infinitely.
There 3 possible szenarios how the existence of the universe could end:
1. Big Rip (the distance between the objects in the universe becomes to lange and the universe bursts like a bubble, this theory seems to be the most probable theory)
2. Big Crunch (where the universe shrinks, like an inversion of the expansion of the universe - maybe combined with a Big Bounce in an oscillating universe)
3. Big Freeze (combined with the heat death of the universe)
From the scientific perspecive, I think that 1 will happen before 3.
A short explanation for the heat death: the driving force for most physical and chemical processes is increasing the entropy in a closed system. The universe can be seen as a huge closed system and the 2nd law of thermodynamics predicts an increase of entropy over the time. Entropy in general can be seen as statistical distribution of matter and energy. When you drive with petrol in your car, the petrol has lots of energy and the entire energy is located in a small area, in the tank. By burning petrol in the motor, you produce carbon dioxide and water steam and the number of molecules after the reaction increases, also the energy which was located in the tank before, is now distributed in the enviroment (as thermal energy).
Same happens with stars: they fuse hydrogen into helium and other heavier elements and they release energy by nuclear fusion. After nuclear fusion, the energy is spread more broad in the room. Sometime in the future, the energy and the particles are spread almost equal in the entire universe. If the energy is equal at any point in the universe, no nuclear fusions will happen and also no chemical reactions will happen, for some reasons.
And at this point in the universe, there is a stand still, a freeze (because no heat energy will wander from a point with high energy to a point with low energy any more), a heat death. The heat death is the logical consequence from the 2nd law of thermodynamics.
The problem with this szenario is, that the expansion of the universe makes the distance between particles larger, so it takes more time to exchange energy between some particles.
Calculations predict that the heat death of the universe will enter in 10^106 years (but you have also to consider proton decay before and during this time frame). But as I mentioned before, I think the decay of subatomic particles and the burst of the entire universe will happen before.
Craig mentions in his books that our universe - if it is a cyclic oscillating universe (scenario 2) - can be maximal in the 10th cycle, because of the increase of entropy (according to some scientists). So also a cyclic oscillating universe seems to have a beginning.
All in all, it is more plausible that the universe had a beginning instead of being infinite.
I gave you lots of scientic arguments to refute an invite universe, in a addition to that I want to give you 3 positive scientific arguments for the premise that the universe had a beginning:
1. Red-light-shift due to the Doppler effect, which shows that objects are moving away from us
2. Cosmic microwave background radiaton, the reason why the temperatur in space is 2 K above absolute zero (0 K)
3. The portion of helium in the universe (~10%) is much bigger than you would expect by nuclear fusion in the stars, the only reasonable explanation for this is nuclear fusion in an early stage of the universe, where the universe was much smaller and much hotter